To make someone appear foolish or stupid, or to reduce someone to the status of a donkey (ass).
From English 'ass' (a donkey, or a foolish person) combined with the verb-forming suffix '-ify'. The term emerged in modern English as a playful but derogatory formation meaning to make someone act like an ass.
While 'assify' never became standard English, it represents the marvelous flexibility of English word-formation—almost any noun can become a verb with '-ify,' which is why we got corporate jargon like 'synergize' and 'optimize.'
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