IPA: /ˈrʌs.lər/
KK: /ˈrʌs.lɚ/
A person who steals cattle or livestock, often associated with the Old West.
The rustler was caught trying to steal cattle from the ranch.
Rustler is derived from the word 'rustle' (from Old English 'rustlian', meaning to make a noise or stir) combined with the suffix '-er' (meaning a person who). A rustler is a person who stirs up trouble, particularly in the context of stealing cattle.
Think of someone who 'stirs' ('rustle') up trouble with cattle — that's why a rustler is a person involved in cattle theft.