Slag

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mid 16th century: from Middle Low German slagge, perhaps from slagen ‘strike’, with reference to fragments formed by hammering. The verb sense ‘criticize’ dates from the 1970s.

wiktionary

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Borrowed from Middle Low German slagge, perhaps from slagen(“to strike”), since originally the splinters struck off from the metal by hammering, from Old Saxon slegi, from Proto-West Germanic *slagi.

See also Dutch slak, German Schlacke, Swedish slagg; also compare slay.

etymonline

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slag (n.)

"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.)).

slag (v.)

"denigrate," by 1971, from slag (n.) in a secondary sense of "worthless person" (1788). Related: Slagged; slagging.