Scottish slang. An act of humiliation/sexual abuse, in which the perpetrator repeatedly slaps their 'boaby
' (Scottish slang for penis) over the victims top lip such that it resembles a 'tache' (or tash
, shortened from moustache).
"Aye David was being a wee dickhead the other night, so three of us pinned him doon and I boaby tached him an absolute belter!"