retreat; withdrawal; depression (hole)
Refers to the act of pulling back or a recessed area. Often used in set phrases like 引っ込みがつかない (unable to back out).
He got himself into a position where he couldn't back out, and had no choice but to apologize.
There is a recess in the wall, and a small shelf is fitted into it.
shy; reserved; withdrawn; introverted
Noun form of the verb 引っ込む (ひっこむ), from 引く (ひく, 'pull') + 込む (こむ, 'go in').