Just like "That's what she said"
When something is not meant to be sexual, but sounds sexual or can be put in to a sexual context.
The origin of the word comes from the TV show "Glee" where it is typically said as a sort of catchphrase by character Santana Lopez.
(taken from show)
"Well you guys have a hole to fill, and I'm just trying to fill it."
"..Wanky"
My bae (sometimes Helenas but usually mine)
"You're dating Kevin Alonso right?" "Yuh yuh." "Isn't he a character from identity v?" "ARF BARJK GRR AARF ARF BGRRRRR BAK BAKR ABARK BARK"