Stun

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Middle English: shortening of Old French estoner ‘astonish’.

wiktionary

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From Middle English stunien, stonien, stounien, from Old English stunian(“to crash, make a loud sound, resound, roar, strike with a loud sound, dash, impinge, knock, confound, astonish, stupefy”), from Proto-Germanic *stunōną, *stunjaną(“to sound, crash, bang, groan”), from Proto-Germanic *stenaną(“to moan, shout”), from Proto-Indo-European*(s)tona-, *(s)tena-(“to thunder, roar, groan”) (compare thunder). Cognate with Middle Low German stonen(“to groan”), Middle High German stunen, stunden(“to drive, push, knock, strike”), Swedish stöna(“to moan, groan”), Icelandic stynja(“to moan”). Related also to Dutch steunen(“to groan; support”), German stöhnen(“to groan, moan”), German staunen(“to be astonished, be amazed, marvel at”), Russian стонать(stonatʹ), стена́ть(stenátʹ, “to moan, groan”). See also Occitan estonar, Old French estoner, English astonish.

etymonline

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stun (v.)

early 14c., "to daze or render unconscious" (from a blow, powerful emotion, etc.), probably a shortening of Old French estoner "to stun" (see astonish). Related: Stunned; stunning.