noun
undershirt worn under a kimono; singlet
A thin, usually white, garment worn directly against the skin under a kimono. The word is a loan from Portuguese 'gibão'.
from Portuguese gibão
着物の下には襦袢を着ます。
You wear a juban under a kimono.
夏は薄手の襦袢が涼しくて良い。
In summer, a thin juban is nice and cool.
Kana spelling is common in everyday writing.
Katakana spelling is sometimes used for emphasis or in informal contexts.
肌着 is a general term for underwear or lingerie, while 襦袢 specifically refers to the traditional undershirt worn under a kimono.
Borrowed from Portuguese 'gibão' (a type of jacket or doublet), adapted into Japanese as 襦袢 (ateji). The exact historical derivation is uncertain, but the word has been used since the Edo period.