"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.)).
"denigrate," by 1971, from slag (n.) in a secondary sense of "worthless person" (1788). Related: Slagged; slagging.
: its purposeful indie-rock slag at commerciality
verb(also slag off) To denigrate; bad-mouth, put down: Everybody was getting slagged/ I don't mean to slag the girls at Douglas/ This time I can't give it to you, can't totally slag you off (1971+)
[origin unknown; perhaps fr German schlagen, ''beat, whip'']