what is the modern equivalent of { talk like an apothecary } ?
To use hard or gallipot words: from the assumed gravity and affectation of knowledge generally put on by the gentlemen of this profession, who are commonly as superficial in their learning as they are pedantic in their language.

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An apothecary is a chemist. Nowadays chemists explain things about your medication quite simply.

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Yes, the old apothecaries were also versed in Latin, and were known for using this language too often (probably when they didn't know what they were saying themselves!).

Today?  I think we'd simply use "Mr (Ms) Know-It-All".