slag n.

Middle English slēn, "strike, beat, strike so as to kill, commit murder," from Old English slean "to smite, strike, beat," also "to kill with a weapon, slaughter" (class VI strong verb; past tense sloh, slog, past participle slagen), from Proto-Germanic *slahanan "to hit" (source also of Old Norse and Old Frisian sla, Danish slaa, Middle Dutch slaen, Dutch slaan, Old High German slahan, German schlagen, Gothic slahan "to strike"). The Germanic words are said to be from PIE root *slak- "to strike" (source also of Middle Irish past participle slactha "struck," slacc "sword"), but, given certain phonetic difficulties and that the only cognates are Celtic, Boutkan says the evidences "point to a North European substratum word."

The verb slēn displays many nondialectal stem variants because of phonological changes and analogical influences both within its own paradigm and from other strong verbs. [Middle English Compendium]

Modern German cognate schlagen maintains the original sense of "to strike."

It is attested by late 12c. as "destroy, put an end to." The meaning "overwhelm with delight" (mid-14c.) preserves one of the wide range of meanings the word once had, including, in Old English, "stamp (coins); forge (weapons); throw, cast; pitch (a tent), to sting (of a snake); to dash, rush, come quickly; play (the harp); gain by conquest."

1915, "cheap, shoddy, or defective goods," from American Yiddish shlak, from German Schlacke "dregs, scum, dross" (see slag (n.)). Alternative etymology [OED] is from Yiddish shlogn "to strike" (cognate with German schlagen; see slay). Mostly commercial at first, by mid-20c. in reference to fiction, movies, television programming, etc. Derived form schlockmeister is by 1953; "purveyor of cheap products," though originally it had a more specific sense in show-biz. 

Ever wonder how these washing machines, toasters, razors, clothes and 101 other items show up on national TV shows? The answer is schlockmeisters!

Nobody is certain where the word comes from but it's the new name applied to a few men in Hollywood and New York who make a Cadillac-style living by giving away their clients' products. The situation is well in hand and a going concern for Al Pretker, Walter Kline and Adolphe Wenland in Hollywood; in Manhattan, there's Waldo Mayo. [Eve Star, "Inside TV" syndicated column, Sept. 10, 1953]

Adjectival form schlocky is attested from 1968; schlock was used as an adjective from 1916. 

"coal dust," mid-15c., sleck, a word of uncertain origin, probably related to Middle Dutch slacke, Middle Low German slecke "slag, small pieces left after coal is screened," which is perhaps related to slagge "splinter flying off metal when it is struck" (see slag (n.)).