petty thief

Describes a thief who done a criminal act in which property belonging to another is taken without that the owner's consent.

Someone who steals for petty reasons, which technically is theft in general, because any authentic justification of theft would disqualify it as theft.

A "real thief" is theoretically, according to this logic, someone who only steals from petty thieves. But a petty thief is just as capable in principal of stealing from a "real thief".