If you suspect a shop stealer, pay this person a lot of attention.
Unclear, but ultimately from shoplifter (which predates the verb shoplift). Structurally, shop steal + -er, but perhaps more likely is that both verb and agent noun forms were coined around the same time by alteration of shoplift / shoplifter to carry the nuanced greater emphasis on theft.
English Wiktionary. Available under CC-BY-SA license.