Origins
Few words in English have undergone as dramatic a moral transformation as 'poison.' Its ancestor, Latin 'pōtiō,' meant simply 'a drink' — any drink, from water to wine. The journey from innocent beverage to instrument of murder is a study in how cultural anxiety can reshape a word's meaning entirely.
The word enters English around 1200 from Old French 'poison,' which itself descends from the accusative form 'pōtiōnem' of Latin 'pōtiō' (a drink, a draught, a dose). Latin 'pōtiō' derives from the verb 'pōtāre' (to drink), which traces to PIE *peh₃- (to drink). The same root appears in Latin 'pōtābilis' (drinkable, whence English 'potable'), in the Greek-derived 'symposium' (literally 'a drinking together,' from Greek 'syn-' + 'posis,' from the same PIE root), and in Sanskrit 'pāti' (he drinks).
The critical semantic shift occurred in Old French. During the medieval period, poisoning became one of the most feared forms of murder — it was covert, deniable, and associated with treachery. Because the most common delivery method was through a tainted drink, 'poison' (originally 'a drink') narrowed to mean 'a drink with something deadly in it,' then broadened again to mean 'any deadly substance regardless of delivery.' The drink disappeared from the meaning, leaving only the death.
Latin Roots
Meanwhile, Latin 'pōtiō' entered English a second time through a more learned channel as 'potion,' retaining the sense of 'a specially prepared drink' — typically medicinal or magical. Thus 'poison' and 'potion' are doublets: two English words derived from the identical Latin source that arrived via different routes and settled into different semantic niches. The doublet split is one of the most striking in the language, with one form acquiring purely negative connotations and the other remaining neutral or even positive.
The parallel with German is instructive. German 'Gift' (poison) derives from the verb 'geben' (to give) — it originally meant 'something given,' a gift or a dose. As in French, the association between receiving a prepared substance and being harmed by it was so strong that the neutral word darkened irreversibly. English 'gift' and German 'Gift' are thus cognates with opposite meanings — one of the most famous false-friend pairs in European languages.
The adjective 'poisonous' dates from the sixteenth century, while the verb 'to poison' appeared almost immediately alongside the noun in the thirteenth century. The figurative extensions — 'poisoning' a relationship, a 'poisonous' atmosphere, 'poison-pen' letters — developed naturally from the word's association with hidden corruption and covert harm. The word 'toxin,' now the scientific term, has its own unexpected origin: from Greek 'toxikon' (poison for arrows), from 'toxon' (bow), because the first poisons the Greeks systematically named were those smeared on arrowheads.