slag (n.) Look up slag at Dictionary.com
1550s, from Middle Low German slagge (German Schlacke) "splinter flying off when metal is struck," related to Old High German slahan "to strike, slay" (see slay).
slag (v.) Look up slag at Dictionary.com
"denigrate," by 1971, from slag (n.) in sense of "worthless person" (1788). Related: Slagged; slagging.