not know what hit one
Definition from Wiktionary, the free dictionary
not know what hit (one) (third-person singular simple present does not know what hit him, present participle not knowing what hit him, simple past did not know what hit him, past participle not known what hit him)
- To be extremely surprised by an event.
- She didn't know what hit her when her boyfriend broke up with her.
- My friend didn't know what hit him when I poured a bucket of water over his head.