Meaning of “bunch (sth) up” - Learner’s Dictionary

bunch (sth) up

phrasal verb with bunch uk us /bʌnʃ/ verb

If material bunches up, or if someone bunches it up, it moves into tight folds:

[ often passive ] My shirt's all bunched up at the back.

(Definition of “bunch (sth) up” from the Cambridge Learner’s Dictionary © Cambridge University Press)