monkey business

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

• noun Silly, mischievous, or deceitful acts or behavior.

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

• noun US, slang Wasting time, or effort, on some foolish project.
• noun An activity that is considered silly, or stupid, or time-wasting.
• noun idiomatic An activity that may be considered illegal, questionable, or a vice, but not felonious.

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

• noun mischievous or deceitful behavior