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.