The original Star Wars underwear, as far as I know, was Underoos ("Underwear that's fun to wear!"). TheSWCA.com has a helpful chart of Star Wars Underoos. Notably, it appears that C-3PO underoos came only in a girl's variety. YouTube user StarWarsFan1975 has posted this Underoos commercial:
The most popular form of skimpy Star Wars wear these days is, of course, the Slave Leia outfit. Maybe it's because I was only ten years old in 1983, but Princess Leia was never an object of my desire, whether clad in a metal bikini or not. The hilarious contrast between what she actually wore in Star Wars and how her character was marketed in posters only serves to underscore, for me at least, that Leia was never sexy. That does not mean, however, that a Slave Leia costume cannot be fetching on the right person, particularly if she is inexplicably covered in baby oil.
The last word, as always, must go to the ubernerds at Wookiepedia. At least one of that site's innumerable contributors felt that no Star Wars encyclopedia was complete without an entry for "underwear". Wookiepedia enlightens us on the topic as follows:
Underwear, also known as undergarments and microgarments, were clothes worn next to the skin, usually under other clothes. They served many functions depending on what species, race and/or culture wore them generally however they were worn to provide support, shape and comfort for the body. It should be noted that underwear was not limited to these uses as some instances saw underwear being used as protective clothes, religious tools, erotic display items or worn simply to protect the modesty of the wearer depending on species, race or culture.
There were many types of underwear worn throughout the galaxy, these included but were not limited to loincloths, body gloves, and brassieres. Nightwear and swimsuits could also be considered "underwear", or more appropriately, "undergarments" due to them being worn next to the skin. Some nightwear and swimsuits even provide the same facilities and supports as underwear and so served the same functions.
Amazing how knowledgeable some people are about Star Wars, isn't it?