also: にゃけ
noun
male homosexual's partner
Archaic or historical term for the passive partner in a male homosexual relationship, particularly in the context of pre-modern Japan. Not used in modern standard Japanese.
江戸時代の文献には、若気という言葉が男色の相手を指して使われている。
In Edo-period documents, the word 若気 is used to refer to the partner in male homosexuality.
noun
Vulgar slang term for the buttocks or anus. Considered crude and offensive in most contexts.
あの男は酔っ払って、若気を触らせた。
That guy got drunk and let someone touch his butt.
The exact derivation is uncertain. The kanji 若 (young) and 気 (spirit/mood) may relate to the historical sense of a young male partner, but the connection to the vulgar sense is unclear.