Green’s Dictionary of Slang

1. (US) a busy, active person.

1872		‘Mark Twain’ Innocents at Home 333: But pard, he was a rustler! You ought to seen him get started once.
1887		M. Roberts Western Avernus (1924) 143: ‘Rustler’ [...] means a worker, an energetic one, and no slouch can be a rustler.
1902		Bisbee Dly Rev. (AZ) 24 Oct. 2/2: he is a better rustler than any competitior. More strict attention to business; longer hours at work.
1909		J.W. Carr in ‘Word-List From Northwest Arkansas’ in DN III:v 403: rustler, n. An active person. ‘He’s a rustler. He made a thousand dollars in three years.’.

2. (Aus./US) one who enjoys a good time.

1885

Bulletin (Sydney) 19 Sept. 22/3: He never refused to go out on a hunt for boss thieves, nor to ante-up his little pile when he bucked the tiger and lost. And he was a rustler when out with the boys.

3. (US) an enthusiast, a promoter, e.g. of a town.

1882

Dly Morn. Astorian 5 Feb. n.p.: Yes, I am satisfied that the Wardner rustler and the Tacoma boomer are full brothers.

4. (US) a man employed in a transient hotel to wake and eject the nightly residents.

1903

Sun (N.Y.) 14 Mar. 12/1: Whin [sic] the rustler wint [sic] to his room to wake him this morning, he was dead.

5. (US prison) a sexual predator.

2000

Other Side of the Wall: Prisoner’s Dict. July 🌐 Rustler: [a sexual] Predator, wolf.