noun
Used for tight-fitting legwear, similar to レギンス. The French origin 'caleçon' originally referred to undergarments, but in Japanese it often means leggings.
from French caleçon
See also: レギンス
彼女はジムでカルソンを履いている。
She wears leggings at the gym.
noun
boxer shorts; long johns
Refers to men's underwear, either boxer shorts or long thermal underpants. The specific meaning depends on context.
寒い日はカルソンをはくと暖かい。
On cold days, wearing long johns keeps you warm.