an event which causes the failure of something that had already started to fail.

I think that argument was the final nail in the coffin of our friendship. (usually + of)

• for sure
• for that matter
• for the asking
• for the best
• for the better
• for the birds
• for the heck of it
• for the hell of it
• for the life of
• for the life of you
• for the long haul
• for the moment
• for the most part
• for the present
• for the record
• for the sake of argument
• for the taking
• for the time being
• for two cents
• forty winks
• for want of a better word
• for what it's worth
• foul-mouthed
• foul play
• foul up