monkey business

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang Silly, mischievous, or deceitful acts or behavior.

Wiktionary

1. n. Wasting time, or effort, on some foolish project.
2. n. An activity that is considered silly, or stupid, or time-wasting.
3. n. An activity that may be considered illegal, questionable, or a vice, but not felonious.

WordNet 3.0

1. n. mischievous or deceitful behavior