slag (v.) Look up slag at Dictionary.com
"denigrate," by 1971, from slag (n.) in a secondary sense of "worthless person" (1788). Related: Slagged; slagging.
slag (n.) Look up slag at Dictionary.com
"refuse from smelting," 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 (v.)).