To put poison into something, or to make something bitter, hostile, or venomous.
From Old French 'envenimer' combining 'en-' (to put into) and 'venim' (venom). The prefix 'en-' was used to create causative verbs meaning to cause something to have a quality.
Shakespeare used 'envenom' to describe words and relationships, not just poison—'envenomed' language has historically been used to describe insults so cutting they seem to carry actual venom.
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