Green’s Dictionary of Slang

1. quickly, unexpectedly, suddenly.

1672		Villiers Rehearsal III i: You shall see ’em come in upon one another snip snap, hot for hot, as fast as can be. First one speaks, then presently t’other’s upon him slap, with a Repartee.
1700		Congreve Way of the World I ii: As soon as your back was turned – whip he was gone! – then trip to his lodging, clap on a hood and scarf, and a mask, slap into a hackney-coach, and drive hither to the door again in a trice.
1709		S. Centlivre Busy Body Act III: To-morrow morning we come slap upon them with a Wedding that nobody thought on.
1730		J. Miller Humours of Oxford I i: Ay, Charles, but to have a Woman slap into one’s Chops at the first Whistle, is losing the Pleasure of the Chase.
c.1750		‘The Black Thing’ in Bold (1979) 24: Then got up his pintle quite stiff for a fling / And ran it slap into my little black thing.
1759		C. Macklin Love à la Mode I i: They wull slaughie off a fellow’s head at one dash, slap.
1795		Burns The Dumfries Volunteers in Works (1842) 221/1: O let us not, like snarling tykes, In wrangling be divided; Till slap come in an unco loon And wi’ a ring decide it.
1799		Sporting Mag. Apr. XIV 40/1: I lost thirty guineas slap.
1814		T. Dibdin Ninth Statue I i: You come slap upon the place at once.
1821		Egan Life in London (1869) 268: His canister came slap against his stand.
1833		‘The Lively Kid’ in Rake’s Budget in Spedding & Watt (eds) Bawdy Songbooks (2011) III 88: Then up a court he bolted slap, / And I’m blow’d if they could hobble him.
1836		T. Haliburton Clockmaker I 159: I went right slap into him, like a flash of lightnin into a gooseberry bush.
1851		‘How Sally Hooter Got Snake-Bit’ in T.A. Burke Polly Peablossom’s Wedding 69: The fust thing she knowed he bit her, slap — the all-firedest, biggest kinder lick!
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor (1968) I 3/1: I had a slice of luck, and paid the 5s. back slap, and got out of it.
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor II 57/2: I went slap into the dog-trade.
1870		B. Harte Luck of Roaring Camp (1873) 31: We’ve had a streak of bad luck since we left Poker Flat, — you come along, and slap you get into it, too.
1885		Bulletin (Sydney) 21 Mar. 5/1: Get ready the melodious tin whistle and the dulcet oboe, not forgetting the ravishing jews-harp, and let him have it slap as he straddles forward.
1891		‘’Arry on Wheels’ in Punch 7 May 217/1: I ain’t one o’ them skinny shanked saps [...] Wot do records on roads for the honour, and faint or go slap off their chump.

2. (also slap off) exactly, perfectly, e.g. slap in the middle.

1759–67		Sterne Tristram Shandy (1949) 297: I threw a fair sheet, which I had just finished, and carefully wrote out, slap into the fire, instead of the foul one.
1821–6		‘Bill Truck’ Man o’ War’s Man (1843) 139: I can answer you for him slap off the reel.
1829		Reading Mercury 6 Apr. 4/5: A pair of out-and-out Kersey Kicksies, got up slap, with pearl buttons.
1832		Egan Bk of Sports 44: He entered the ring slap up to the mark.
1845		J. Lindridge Sixteen-String Jack 197: Vell, that vud be out and out slap, shiver my blinkers!
1852		‘The Cadger’s Ball’ in Farmer Musa Pedestris (1896) 149: Had on a pair of pillow cases, / Transmogrified slap into ducks!
1865		Dickens Our Mutual Friend (1994) 573: ‘The money must be paid.’ ‘In full and slap down.’.
1877		J. Diprose London Life 44: I’m blow’d if he ain’t gone slap into the ditch.
1900		Bulletin (Sydney) 15 Sept. 25/2: Slapofski is a fine, confident sort of a name that appeals to this continent slap off, so to speak.
1909		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:v 371: slap, adv. Entirely. ‘We worked till slap dark.’.
1939		R. Chandler ‘Pearls Are a Nuisance’ Spanish Blood (1946) 121: Sunset goes slap through it.
1949		Wodehouse Mating Season 45: Slap on top of that having to dine in a strange house without dressing.
1962		R. Cook Crust on its Uppers 23: Parking [...] slap under a no-parking sign.

3. directly; completely.

1829		Marryat Life of Frank Mildmay I 103: I, and my Noah’s Ark, lay slap in the way.
1838		Dickens Oliver Twist (1966) 211: Slap through the town [...] there’ll be nobody in the way, to-night, to see us.
1848		G.W.M. Reynolds Mysteries of London II (2nd series) 155: I told him slap as how two gentlemen live here.
1851		Portage Sentinel (Ravenno, OH) 7 July 1/4: I got him [i.e. a horse] into Washington Street [...] but, jes’ op’site the Old South, he fell slap down.
1860		Hotten Dict. of Modern Sl. etc. 217: slap exactly, precisely; ‘slap in the wind’s eye,’ i.e., exactly to windward.
1873		Sl. Dict.
1881		‘’Arry on Woman Rights’ Punch 2 Apr. 156/1: Oh, I’d suffridge ’em! Slap agin Nature, you know, wrong end huppards, in short.
1890		Clarkesville Eve. Tobaco-leaf Chron. (TN) 7 Mar. 4/3: Alderman Wake had his pocket slap full of pavement ordinances.
1908		H. Lawson ‘The Song of the Back to Front’ in Roderick II (1967–9 ) 240: And the fattest coward in England’s tale brought tucker slap through the French.
1929		J.B. Priestley Good Companions 55: ‘Never hesitate a moment. Go slap into business.’ Mrs. Chillingford said this with immense gusto, then went slap into a piece of sandwich cake.
1934		H. Miller Tropic of Cancer (1963) 18: I stand her up, slap up against the wall, and I try to get it into her.
1948		Z.N. Hurston Seraph on the Suwanee (1995) 899: I’m crossing this bar if I land you slap in Hell! Pray!