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 *slahan, from root *slog- "to hit" (cf. 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 from PIE root *slak- "to strike" (cf. Middle Irish past participle slactha "struck," slacc "sword").
Modern German cognate schlagen maintains the original sense of "to strike." 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."
"instrument on a weaver's loom to beat up the weft," Old English slæ, slea, slahae, from root meaning "strike" (see slay (v.)), so called from "striking" the web together. Hence the surname Slaymaker "maker of slays."
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+)