Knowing a subject deeply, and usually because you have spent time with it — read it, practised it, worked in it. It is not just 'has heard of'; someone well-versed in a topic can talk about the details and handle the awkward cases. It almost always comes with 'in': well-versed in tax law, well-versed in Taiwanese history, well-versed in the software. You can say it of practical skills too, not only book knowledge. The word feels formal and respectful — you meet it in job ads, introductions, and news writing rather than in chat with friends. It is often used slightly loosely in job ads, where 'well-versed in Excel' may only mean 'can use it competently', so the strength of the claim depends on who is speaking. It works as a description after 'be' ('she is well-versed in...') and before a noun only rarely.