People who steal cattle or other livestock, especially in the American Old West, or people who make soft whispering sounds.
From 'rustle,' an onomatopoeia imitating soft sounds, combined with the agent suffix '-er.' In the context of cattle theft, it evolved during the 19th century American frontier when livestock theft was rampant.
The word 'rustler' is brilliantly onomatopoetic—the soft 'shhh' sound mirrors how cattle thieves allegedly moved quietly through the night, making the word itself sound like the crime! This shows how English sometimes encodes meaning directly into its phonetics.
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