"Believe you me"

Every time I hear this phrase, I'm left a little puzzled. I think I know what they are trying to emphasize, but is this correct? And if it is acceptable, would a phrase like that be left alone or would copyeditors replace it with a more commonly used phrase?

Lily's Question about Idioms

I found that The Free Dictionary has a section on idioms. You can type into the search box the idiom you want to look up, and the meaning will appear on a new page.

role of thump

Is there a reference book that could be referred for cliches or idioms. If the author has phrased a metaphor incorrectly, is there a way to look it up?
Saying: Toe the line.
Error: Tow the line.