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1.
To get an assassination medal in halo 3 by meleeing in the back. Commonly used by tsquared captain of the halo 3 team Str8 Rippin
Tsquared: Give em' the back smack baby!
by BoweTheShow June 22, 2009
 
2.
action verb: To strike another person because they have just pissed you off. One can backsmack by thought, word, or deed.
Steve: "Did she just cut you off?"
John: "Yeah, I 'd like to backsmack her!"
by Lexie 1912 September 19, 2008
 
3.
To violently slap someone on the back.
"Dude, it hurts so bad!"

"Take that back smack bitch!"

"Did you see Todd totally get fucked up, that was the worst back smack I have ever seen!"
by Tango09 March 31, 2010
 
4.
To talk about someone behind their back
Brittany:"wow she looks really ugly"
Jessica:"sorry I don't backsmack"
by Backsmack September 12, 2014