It does sound that way depending on how close you are to that person (friendship wise). But I would rather say. "Could you hold on for a second." :)

I agree with Richard - adding "please" always helps. Even better, use the "Could you ____, please?" pattern. "Could you wait (one second / a moment), please?" "One moment, please." (I typically use this.)

Well, would it be impolite in Chinese to say "等!"? Wouldn't it be more polite to say "等一下" or "請等一下" In English, we might say "wait a minute please" or "hang on a sec" ["sec" = "second (of time)" ] If the person is doing something (such as walking or doing some kind of work), you might say "hold on a sec" or "hold up". But using the word "please" is always more polite.