So Manic Hispanic gets the prize for asking it first. . . because I know there were others who wanted to.

"I heard mormons get a full body undergarment (we called them mormon undies in high school) after their mission. That one is suppsed to wear them at all times and if they show then the clothes you are wearing are not modest enough.
wouldn't they start to smell if you always wore them?
did you get a pair?
is this a load of horse hockey?"

Like all good myths, this one has its origins in truth, and so we must begin at the beginning.

So lets start out with a mini discusion of the word "sacred." Can anyone give me a defenition? Anyone? Yes, you in the wings and halo, thank you.

According to dictionary.com:

1.	devoted or dedicated to a deity or to some religious purpose; consecrated.

2.	entitled to veneration or religious respect by association with divinity or divine things; holy.

3.	pertaining to or connected with religion (opposed to secular or profane): sacred music; sacred books.

4.	reverently dedicated to some person, purpose, or object: a morning hour sacred to study.

5.	regarded with reverence: the sacred memory of a dead hero.

6.	secured against violation, infringement, etc., as by reverence or sense of right: sacred oaths; sacred rights.

7.	properly immune from violence, interference, etc., as a person or office.

Yes, thank you, a very complete answer.
Now, I know what you are thinking, "Hello, Duchess, we know what the word sacred means, we are mostly college educated and aren't totally illerterate." I know, I know. I just wanted to be clear. I'm sure all of you also know that our society basically doesn't understand the concept of sacred, even if they can define it. Evidenced by the common question "Is nothing sacred anymore?"
And my answer, well yes, but only to those individuals who hold things sacred, and therefore, if you hold something sacred, you would be smart, and entirly appropriate to keep that thing protected and keep it within the sphere of others who also hold it sacred. Perhaps you may have read or heard the Biblical term for this, we are told to not "cast our pearls before swine." Now it's pretty clear why you wouldn't take a beautiful strand of pearls and throw them into a pig pen. It's not that there is anything wrong with the pig, it's just doing what pigs do. It wallows in mud, eats, sleeps, lives a good pigs life, but it wouldn't appreciate the pearls, and while it might find them momentarily interesting, it would not know what to do with them or how to care for them.
OK, so now that we know all about pearls and pigs, lets talk a little about LDS temples. If you live in the US you have likely seen an LDS temple. They are stunning and magnificent buildings with luxourious grounds. There is one here in Bellevue, if you haven't seen it, take a drive over, it's a beautiful building and the Duke's dad helped to build it when he was a baby.
You may have also seen our somewhat plain chapels, where we go to church on Sundays. We don't attend the temple on Sundays, they are open Tues-Sat, generally. So what do we do in the temples?
The temples are of the greatest importance to our religion. Inside of them is where we participate in ceremonies, (simple in nature, no puppies, no chickens, no wierdness) to seal our families together for eternity. For instance, the Duke and I were married in the Salt Lake City temple, but the ceremony contains no reference to "death do you part." Instead, you covenant with one another to form a marriage, a bond, a family that will last through eternity. We believe that families can remain intact in the following life, if certain criteria are met. In addition to sealing ceremonies we learn what those criteria are and how we are to meet them, then we make promices both to our families and to God that we will do those things. So the main thing that we do in the temples is learning.
We don't talk a lot about what we do in the temple because it is so sacred to us, as it deals with both our God and our families. It is also one of those experiences that you can't really understand until you've done it yourself, you know, like bungee jumping, so it's really pointless to talk about it with people who haven't experienced it.
So, that's all well and good, but we aren't able to go to the temple everyday, and usually not even once a week. There is not perscribed number of times you should go, but once a month is common. So, in the meantime, how do we remember the promices that we have made to God and to our spouces (and children.)
Humans have an uncanny ability to forget really important things. If this wasn't so, no husband would ever forget his anniversary. Some other case-in-points: people often forget, in the day to day grind, why they originally fell in love with thier partner. People forget to thank one another for all sorts of things. People forget how much they love their children.
And so, I finally get to the answer to the question. Once you go through the temple for the first time, this is usually right before you either serve a mission or get married, you are given special clothing to wear to help you remember the promices that you have made in the temple. We refer to them as garments, usually, although the official name is "garments of the holy priesthood." We wear them under our clothes and they consist of what looks like a tee shirt (or more of a baby-tee for women) and shorts (or more like boxers for men.)
We always keep them covered because they are sacred and personal. They are not "full body" undies, as many of you have seen me wear baby tees and knee length skirts. I assure you that I was wearing them, because I always do. They don't get stinky because you can buy as many pairs as you want, and it is perfectly legit (and in fact expected) to throw them in your washing machine. They come in different cuts, sizes, fabrics, lots of options. It is also perfectly acceptable to take your garments off for activites that they are not appropriate for, such as swimming, water skiing, going to the beach, etc. We wear them whenever it is possible, but always keep them covered, and take them off entirly if we can't keep them covered.
They act in much the same way that the yarmulka, prayer belt, and black clothing can for traditional Jews, they remind us of our covenants and our convictions.

In additionn, without making light of something sacred, they do have other benefits:
1. I never have a panty line.
2. I never have a wedgie.
3. I don't need to wear a slip in most cases.

So there you go Manic Hispanic, the truth about "Mormon undies."

Please forgive me for spelling mistakes, blogger's spell check doesn't appear to be working.