O.E. slean "to smite," also "to kill with a weapon" (class VI strong verb; past tense sloh, slog, pp. slagen), from P.Gmc. *slakhanan, from base *slog- "to hit" (cf. O.N., O.Fris. sla, Dan. slaa, M.Du. slaen, Du. slaan, O.H.G. slahan, Ger. schlagen, Goth. slahan "to strike"), from PIE base from base *slak- "to strike" (cf. M.Ir. pp. slactha "struck," slacc "sword"). Modern Ger. cognate schlagen maintains the original sense of "to strike." Meaning "overwhelm with delight" (1340) preserves some of the wider rangeof meanings that the word once had, including also "to strike a spark" (O.E.).
early 13c., from O.E. (ge)slegen, pp. of slean (see slay). The noun meaning "Those who have been slain" is attested from mid-14c.
To impress someone powerfully, esp to provoke violent and often derisive laughter: Pardon me, this will slay you/ The boys who slay me are the ones who have set pieces to recite when they answer the phone(1593+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source