play (your) ace

to do the thing that you know will bring you success.

The prosecutor played her ace, the results of the DNA tests on samples taken from the victim's clothing.

• bush league
• bush telegraph
• business as usual
• business end
• business end of
• business end of something
• business is business
• busman's holiday
• bust a blood vessel
• bust a gut
• bust a gut laughing
• bust ass
• bust balls
• bust your ass
• bust your balls
• but not in the biblical sense
• but on the other
• but on the other hand
• but still and all
• butter someone up
• butter up
• butter wouldn't melt in mouth
• butter wouldn't melt in someone's mouth
• but that's another story
• but then again