I didn't strike him with a stick. OR: It wasn't a stick with which I struck him.
Comment:
The first translation more likely represents the sense because the narrative up to this point does not mention that the speaker struck his attacker, whereas it was his attacker who hit him. So it is more probable that the speaker is here denying any allegation that he hit his attacker. In both cases, laau designates an instrument used in an act of hitting.