Green’s Dictionary of Slang

1. (US Und.) involved in criminality.

1859		Calif. Police Gazette 10 Apr. 4/4: ‘Well, Harry, [...] its been running in my mind that you’re on it.’ ‘On what?’ ‘Oh, on some “lay” or other.’.
1873		J. O’Connor Wanderings of a Vagabond 277: ‘They’re all on it.’ ‘On it! On what? ‘demanded the Major. ‘The rob,’ laconically replied Mr. Chapin.
1943		R. Llewellyn None But the Lonely Heart 21: Shoplifting ain’t thieving, and that’s it [...] I’ve been on it for years.

2. ready, prepared, capable of, skilled in, in control; thus (US black) to be on it like a hornet.

1833		Sweet & Knox On a Mexican Mustang 145: Major, you don’t mean it. You don’t know who I am. I’m on it, I want you to know. I’m no feather-bed soldier.
1847		Carleton & Harvey Black Prophet 109: ‘I see that Hanlon, I’ll tell him you want him, Masther Richard; an’ now that I’m on it, I had betther say a word to you before I go.’.
1862		‘Artemus Ward’ Artemus Ward, His Book 96: ‘Do you bleeve in Solomon, Saint Paul, the immaculateness of the Mormin Church and the Latterday Revelashuns?’ Sez I, ‘I’m on it!’.
1872		‘Mark Twain’ Innocents at Home 334: Pard, he was on it. He was on it bigger than an Injun.
1890		‘Doss Chiderdoss’ ‘The Rejected’ Sporting Times 29 Mar. 1/3: Upon any subject that he chanced to light on / Whether epigram or sonnet, he was on it, fairly on it.
1900		Bulletin (Sydney) 27 Jan. 24/1: The match was the outcome of a dispute on the Moorfield racecourse between a well-known trainer-owner and the jock. in question as to whether Skein Dhu was ‘on it’ that day or otherwise.
1900		Marvel XIV:343 June 15: I’m on it [...] but what do we get for the job?
1976		D. Ireland Glass Canoe (1982) 139: The whole pub was on it.
1986		C. Stroud Close Pursuit (1988) 100: I be on it, man.
1997		Simon & Burns Corner (1998) 349: ‘Okay, Chief,’ says Gary, [...] ‘I’m on it.’.
2001		G. Pelecanos Right As Rain 8: You’re always on it. I don’t know why I feel the need to remind you.
2008		J. Niven Kill Your Friends (2009) 5: I’m on it, Steven. relax.
2016		A. Wheatle Crongton Knights 66: ‘You’re not gonna flop out of our mission tomorrow, are you?’ [...] ‘No man [...] I’ve got your back. I’m on it’.
2020		G. Krauze What They Was 11: Snoopz, I know you’re on it [i.e. a robbery].

3. (Aus.) in specific use of sense 1, fixing a horse-race.

1901

Sydney Sportsman (Surry Hills, NSW) 6b Feb. 4/1: It is an open secret that certain strong stables owe their security from searching inquiry to the fact that whenever their horses are ‘on it,’ many of the officials have their ‘bit’.

4. (US campus) good, likeable.

1987

Eble Campus Sl. Oct. 5: on it – good acceptable: ‘I like Cheryl’s dress.’ ‘Yeah. It’s really on it.’.