Knickers is actually
a standard word for underwear, mainly in Britain. "Knickers" derives
from "knickerbockers," or "loose-fitting short pants gathered at the
knee." Because the city's early Dutch settlers wore those pants,
"New Yorkers" became known as "Knickerbockers." The Knickerbockers,
more commonly "The Knicks" is the name of New York's NBA team.
In the 1500s, a corselet was something a soldier might wear,
a piece of armor for the torso. The word comes from the French word
for body. Several centuries later, the same word emerged and
shortened to corset, to describe a combination of girdle and
brassiere.
Drawers does not refer to where you store them, as in a
chest of drawers. The word drawers has been used since the 16th
century to refer to garments such as stockings, underpants, and
pants. It comes from the verb draw used in the sense of pull, likely
because you pull them up your legs.
The union suit gets its name by uniting the upper and lower
pieces of underwear in one garment. Two-piece long johns are
more common these days, and do not require a seat flap. Long johns
are reputedly named after the late-19th-century heavyweight boxer
John L. Sullivan, who wore a similar-looking garment in the ring.
This explanation is uncertain and the true origin is unknown.
Singlet usually describes a sleeveless undershirt. It also
refers to the one-piece suit a wrestler wears. It has only one
thickness of cloth. A doublet is not underwear, but a lined
jacket worn by men during the Renaissance.