1. A devious, scheming, and untrustworthy person, group, organization, etc. You're working for Brett Thompson? Watch out—that guy's a slippery customer. Companies like this are slippery customers, coming up with all sorts of elaborate means of getting around regulations.

2. Someone who is difficult or impossible to apprehend or pin down due to their cunning. The notorious criminal has proven to be a slippery customer for police, eluding capture once again.

3. Someone or something that is difficult or tricky to determine or define with certainty. Modern art is often a slippery customer because it defies the boundaries of what a lot of people consider "real" art. What some may consider utterly brilliant others will stare at and wonder why a seven-year-old couldn't have done better.