In a manner resembling or characteristic of a goat; in a goat-like way.
From 'goatish' + '-ly' (Old English adverbial suffix converting adjectives to adverbs).
This is a great example of how English builds meaning modularly—take any adjective, add '-ly', and boom, you have an adverb. Medieval writers could say someone 'goatishly clambered' up a hillside!
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