In law enforcement, the phrase gypsy cop is slang

Slang

Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often to be found in areas of the lexicon that refer to things considered taboo...

 for an itinerant lawman, a peace officer

Peace officer

A law enforcement officer , in North America, is any public-sector employee or agent whose duties involve the enforcement of laws. The phrase can include police officers, prison officers, customs officers, immigration officers, bailiffs, probation officers, parole officers, auxiliary officers, and...

 who floats from department to department regardless of, or because of, misconduct or poor job performance.

The phrase entered public parlance after the infamous Tulia drug stings

Sting operation

In law enforcement, a sting operation is a deceptive operation designed to catch a person committing a crime. A typical sting will have a law-enforcement officer or cooperative member of the public play a role as criminal partner or potential victim and go along with a suspect's actions to gather...

, where itinerant lawman Tom Coleman allegedly set up innocent people, most of them black, as part of a long-term undercover operation.