Refers to a thief who steals small items stealthily, often in crowded places or from homes without being noticed. The prefix 小 (こ) emphasizes the petty or sneaky nature.
I had my wallet stolen by a sneak thief on the train.
A sneak thief steals small items without being noticed.
Compound of 小 (こ, 'small, petty') and 盗人 (ぬすびと, 'thief'). The reading ぬすびと is a native Japanese word for thief, and the prefix こ adds the nuance of sneakiness or pettiness.