slyrapped

An unexpected strike; an ambush. A blow delivered when the recipient is not looking. (ex.) We were arguing over nothing when I turned and he slyrapped me.

It is a portmanteau(blend) from sly meaning sneaky or underhanded and rap meaning to hit or wallop or strike.

As for it's origins it's been in my lexicon all my life. I really can't be specific other than say it is street jargon rememberedfrom my days growing up on the streets of The Bronx, NY.