one by one; thoroughly; exhaustively
Describes doing something meticulously, covering every single item or possibility without skipping any. Often used with verbs like 調べる, 探す, or 当たる.
The police searched the area exhaustively, going door to door.
I checked the documents with a fine-tooth comb for any mistakes.
Literally 'in the manner of squashing lice', from 虱 (louse) + 潰し (crushing) + に (adverbial particle). The image is of methodically crushing lice one by one, hence doing something exhaustively and without missing any.