More drunk or more containing alcohol; the comparative form suggesting a greater degree of intoxication or alcohol content. A boozier party is one with more drinking happening.
From 'boozy' (from 'booze' + the adjective suffix '-y', Old English '-ig') + the comparative suffix '-er' (Old English '-er'). The form emerged as 'boozy' became established in English.
The progression from 'booze' (noun) to 'boozy' (adjective) to 'boozier' (comparative) shows how English lets you make related words from the same root—you can talk about the thing, describe it, and compare it! This flexibility is one reason English has so many words.
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