onto, adj. — Green’s Dictionary of Slang

onto adj.

1. (also on to) aware of, esp. of someone’s supposedly secret or underhand plans.

1877		Chicago Street Gazette 20 Oct. n.p.: And not be trying to put in so much style around the St Mark’s Hotel, for very near all of the boys are on to you; and when you register, please leave the word ‘New York’ out, for we know it’s from the Bridewell you are.
1883		G.W. Peck Peck’s Bad Boy and His Pa (1887) 113: O, give us a siesta. I am on to you bigger than an elevator.
1896		Ade Artie (1963) 25: I’m on to you bigger ’n a house.
1907		‘Hugh McHugh’ Beat It 63: I was on to the Count Cheese von Cheese the moment he opened his talk-trap.
1915		Van Loan ‘The Spotted Sheep’ in Taking the Count 99: That’s [i.e. a fixed prizefight] almost as bad as a fake foot race. Everybody in the world is on to it.
1923		Wodehouse Leave it to Psmith (1993) 502: I’m on to you, Bill!
1936		Auden & Isherwood Ascent of F6 I ii: I must congratulate you, Ransom. You’re on to a big thing – a big thing for all of us!
1940		M. de la Roche Whiteoak Heritage (1949) 116: ‘So you’ve got on to the racket too!’ ‘What racket?’ ‘That she doesn’t open her door to others when she has her favourite visitor inside.’.
1953		T.A.G. Hungerford Riverslake 208: Lamp old Uncle – he’s a goner this time. The Bastard’s right onto him.
1957		C. Himes Imabelle 86: Now that the gang knew the police were on to them.
1965		C. Himes Rage in Harlem (1969) 85: [as 1957].
1973		F. Huelin ‘Keep Moving’ 34: [T]he johns were on to me.
1976		P. Theroux Family Arsenal 258: I heard he’d been rumbled. The Yard’s on to him.
1981		J. Ellroy Brown’s Requiem 74: For the first time in my life I was on to something important, something vast and complex, and I was the sole arbiter of it.
1999		K. Sampson Powder 234: That, m’boy is one of the surest signs yet that we’re on to something here.
2001		K. Sampson Outlaws (ms.) 4: The more we done it, the more I realised what we was onto.

2. harrassing, nagging.

1886		Lantern (N.O.) 8 Sept. 3: Remove this debris or the Lantern will be on to it incessantly.
1905		H.G. Wells Kipps (1952) 69: Shalford [...] had been rather ‘on to’ him for the past few days in an exceedingly onerous manner.
1924		Lawrence & Skinner Boy in Bush 49: Gran is on ter me right cruel if I don’t see to it.
1974		D. Ireland Burn 8: Christ, everyone’s onto me. All on my back.

In phrases

put someone onto (v.)

1. (orig. US) to introduce a topic, to point out, esp. to point out a chance of possible social or financial gain etc.

1702		Cibber She Would and She Would Not IV i: The old Gentleman is substantially convinc’d that ’tis you who have put Don Philip upon laying this pretended Claim to his Daughter.
1887		Bulletin (Sydney) 22 Oct. 8/4: The cash was immediately drawn, the black mailer despoiled of his £10 commish. and now it’s very dangerous to ask this very highly respected citizen whether he can ‘put you on to making £40 for a slight consideration!’.
1890		Sporting Times 19 Apr. 6/1: Told him when I knew something I would put him on a good thing.
1903		A.H. Lewis Boss 372: When you ask me can I locate our party, I says to myself, ‘Sure thing!’ and I’ll put you on to why.
1910		Sporting Times 8 Jan. 2/4: Take my tip, now, an’ I’ll put you on a good thing!

2. to introduce someone, to give access to someone.

1951

Cusack & James Come in Spinner (1960) 24: Hero be blowed. He’s a choco. Caught in the draft. They say her old man put the military on to him but she’s succeeded in keeping him in Melbourne so far.