Poisonous

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Middle English (denoting a harmful medicinal draught): from Old French poison ‘magic potion’, from Latin potio(n- ) ‘potion’, related to potare ‘to drink’.

wiktionary

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From Middle English poisounous, poysonouse, equivalent to poison +‎ -ous.

etymonline

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poisonous (adj.)

"having the properties of a poison; containing poison," 1570s, from poison (n.) + -ous. Failed rivals were poisonsome (1590s), poisonful (1550s), poisony (1590s). Earlier poisoned was used (late 15c.). Related: Poisonously; poisonousness. Compare venomous.