Whacking the gavil is when you have a female participant upside down and legs open so she presents herself as a judge's bench. Then a man opens his pants and slaps his penis on her vagina.
Oh man last night youll never guess that this one whore let me start off sex by whacking the gavil!
Popularized by Gavin Ng, it is when you are able to chat up with someone and smoothly communicate with them without prior knowledge and using only your intuition.
Wow, how'd you get that internship. Oh I justused the Gavin Effect