dirty laundry

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Personal affairs that could cause embarrassment or distress if made public.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Unflattering facts or questionable activities that one wants to remain secret, but which some other may use to blackmail with.
• noun Laundry that is unclean or soiled
• noun slang A clothes hamper or other container used to place unclean or soiled laundry in.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun personal matters that could be embarrassing if made public