Green’s Dictionary of Slang

(a) (gambling, also snoozer) a stake that has been left on the table, when neither the croupier/banker nor a winning bettor has realized it is to be picked up.

1859		Matsell Vocabulum 117: sleeper A bet won by the bank or a better, which has been overlooked and lies on the table without a claimant.
1876		Butte (MT) Miner 8 June 1/3: P’raps you don’t call to mind the night when you and me snatched a fifteen-dollar sleeper when a drunk didn’t savy enough to pick up a split on a bet o’ thirty. [...] P’raps you and Jake Small ain’t snacks in ropin’ in snoozers?
1890		Sporting Times 8 Mar. 2/1: We perceived that our stake was ‘a sleeper,’ that is, the croupier had been too occupied to collect it, and so it was left down for the next whack, which was to be the last.
1896		H. Blossom Checkers 50: I [...] stacked his chips and saw that no one pinched his sleepers.
1897		W.A. Sun. Times (Perth) 19 Dec. 4/6: How Jones thought he had a ‘sleeper’ for £40 (also his attempt to make the ‘sleeper'’ safe) is quite another story.
1915		Drew & Evans Grafter (1922) 25: ‘[H]e’d stand up behind the players, broke to the world, keeping his eye out for “sleepers”’.
1933		Ersine Und. and Prison Sl.

(b) (US) a bar customer who was too drunk to pick up their change when they left; the bartender kept it.

1924

O.O. McIntyre New York Day by Day 22 Jan. [synd. col.] In the old days of hotel bars cashiers would work without pay just for the revenue derived from what were known as ‘sleepers’ – those who became rather befuddled and walked away without taking their change.

(c) (US) a prostitute’s unexpected customer, picked up in a bar at the end of the night.

1971

Winick & Kinsie Lively Commerce 173: Women often go to a bar about an hour before closing time to see if they can find a ‘sleeper’ or unexpected customer.

(d) (Aus. gambling) a note that has been overlooked in a bookmaker’s counting.

2013

T. Peacock More You Bet 64: A ‘lazy’ or overlooked or forgotten 50 or 20 note that had escaped ‘the count’ [is] referred to as a ‘sleeper’.

(a) a potentially successful racehorse that has eluded the eye of the betting public.

1899–1900		C.L. Cullen Tales of the Ex-Tanks 364: When I’d dig a sleeper up, and get it right [...] the word got out every time.
1917		Van Loan ‘The Redemption Handicap’ in Old Man Curry 186: A sleeper woke up on me. Four bets down and not a single bean.

(b) (orig. US sporting) any product that gains acceptance and success only slowly; also attrib.

1892		Outing (NY) Mar. 454/2: Williams won the high and low hurdles in record time, [...] and Harmar a second in the mile, being beaten by Wells, a ‘sleeper’ from Amherst .
1903		J.P. Paret Lawn Tennis 350: Sleeper, a slang expression meaning a player who is much better than was thought.
1910		Sun. Times (Perth) May 1st sect. 1/1: They Say [...] That the connections of an unregistered racing stable were in the soup last Wednesday, That knowing a dangerous opponent was a ‘sleeper’ they went for a win on their own. That they disposed of the dead ’un easily enough, but did not allow for another.
1926		‘Und. and Its Vernacular’ in Clues mag. 158–62: sleeper Something of value that has been overlooked.
1944		W. Winchell On Broadway 27 Sept. [synd. col.] A ‘sleeper’ song hit ‘Mexican Hayride’ is becoming a click in the swankiest spots.
1947		A. Baer in Waterloo (IA) Daily Courier 19 Jan. 35/1: A sleeper is something that sneaks up on you and scores without publicity.
1950		Zanesville (OH) Signal 16 July sect. I 5/1: The sleeper went over and the Spanish national ensign went up at San Diego.
1972		R. Barrett Lovomaniacs (1973) 21: David Strauss is in a picture the guinea financed for a tax loss and instead it’s a sleeper!

(c) (US) a financial collapse.

1908

K. McGaffey Sorrows of a Show Girl Ch. xii: He got stung on a sleeper, and had to hock the family jewels.

(d) a surprise.

1953

F. Paley Rumble on the Docks (1955) 219: Gotham was sure now that Jimmy was a real sleeper.

(e) one whose character has yet to be properly developed.

1993

K. Scott Monster (1994) 253: He was a sleeper who just needed someone to coach that ruthlessness out of him. Once I’d tapped into it, he roared to life.

(f) anything that is currently postponed from investigation.

2010

K. Richards Life 7: The sleeper was Freddie’s briefcase, which was [...] as yet unopened and we knew he had cocaine in there.

(a) any form of barbiturate sleeping pill.

1960		(con. 1930) J. Havoc Early Havoc 240: Scotty slipped a whole load of sleepers into that bottle.
1961		Rigney & Smith Real Bohemia 63: The barbituates (’sleeping pills,’ goof balls,’ ‘sleepers’).
1970		Times 18 Aug. 6: I’ve come into work bombed on sleepers a couple of times (they make you go round like you’re drunk).
1977		(con. 1960s) Nicholson & Smith Spend, Spend, Spend (1978) 169: I took a gin up to my room and a hot cup of tea and a sleeper.
1988		J. Healy Grass Arena (1990) 118: That was when I realized they’d slipped him some sleepers.
1996		P. Howard The Joy (2015) [ebook] ‘I’m after taking a load of sleepers’.
1996		J. Cameron It Was An Accident 161: Then again maybe it was the little sleepers the doc gave me.
2002		J. Cameron Hell on Hoe Street 77: So I was alone in Pakistan with the lurgy and a television and a few sleepers.
2023		R. Milward Man-Eating Typewriter 245: ‘Turns out the old zot’s got a skimpy tolerance for sleepers’.

(b) heroin.

2001

ONDCP Street Terms 19: Sleeper — Heroin; depressants.