noun
armpit; underarm
Standard anatomical term for the area under the shoulder joint. 腋の下 is a rarer spelling.
脇の下に汗をかいた。
I sweated under my arms.
体温計を脇の下に挟んでください。
Please put the thermometer under your armpit.
Standard kanji spelling.
Alternate kanji 腋; less common than 脇.
Mixed kana-kanji spelling, common in everyday writing.
脇 alone can mean 'side' or 'armpit' depending on context, but 脇の下 specifically refers to the armpit area.
Compound of 脇 (わき, 'side, armpit') + の (possessive particle) + 下 (した, 'under'). Literally 'under the side/armpit'.