“Fifth column” means a hidden group inside a country or organization that secretly helps an outside enemy or rival.

Yacht looks mismatched because English borrowed it from Dutch, kept much of the spelling, and gradually changed the pronunciation.

Conceit is an elaborate literary comparison. Concept is a general idea or plan. Here is how to tell them apart quickly.

Nauseated usually means feeling sick. Nauseous traditionally means causing nausea, though many speakers now use it for feeling sick too.