In a manner resembling an ass; stubbornly or foolishly.
From 'assish' plus '-ly' (in the manner of). This double suffix construction (from 'ass' → 'assish' → 'assishly') shows how English builds adverbs through layered modifications.
English lets us stack suffixes in ways that would be impossible in Romance languages—we can turn a noun into an adjective into an adverb in one word, creating increasingly specific meanings with each layer.
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