Idiom Definition:
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"to talk through your hat"
to talk as if you know a subject thoroughly when you really don't know what you are talking about
Synonyms:
babble, baloney, blather, blurt, bosh, bull, bunk, chatter, crap, drivel, gab, gabble, gibberish, guff, hogwash, hokum, hooey, jabber, malarkey, nonsense, poppycock, prattle, rubbishyak, yakety-yak