Green’s Dictionary of Slang

1. directly, straight at, immediately.

1851		‘Bainbridge’s Tid-Re I’ in Jack Tar’s Songster 17: Our gunner [...] touched off our Yankee barkers so neatly in time, that slap dab every shot struck her somewhere.
1886		Turf, Field and Farm XLII. 174/3: He was goin’ that fas’ he run slap-dab agin me afo’ he seed me [OED].
1896		DN I 399: Slap-dab [...], violently or awkwardly. ‘He rushed in slap-dab and broke things.’.
1897		Leaf-Chron. Wkly (Clarksville, TN) 16 July 4/2: So at last he puts himself ‘slab-dab’ on the Kentucky constitution.
1900		Marvel 15 May 15: In a couple of days [...] they were at sea right bang slap into the middle of the ocean.
1907		Austin American (TX) 25 July 6/4: We keep telling you, keep the train slab-dab on the track.
1912		R.W. Brown ‘Word-List From Western Indiana’ in DN III:viii 590: slap-dab, adv. Squarely. ‘He ran slab-dab into a big fat woman.’.
1919		Marvel 1 Mar. 7: I’ll be around bang-slap on time.
1925		Gaffney ledger (SC) 11 June 5/3: The car swerved out of the road and ran slab-dab agin the front of the tavern.
1931		D. Runyon ‘Breach of Promise’ in Runyon on Broadway (1954) 21: I run the car slapdab into a stone gatepost.
1937		McComb Dly Jrnl (MS) 17 June 2/2: Someboyd smacks him slab-dab in the mouth and tells him to shut up.
1940		Star Trib. (Minneapolis, MN) 13 mar. 15/1: He ran slab-dab into a fire in a hollow of one of the hills.
1948		Essex Newsman 22 Oct. 3/6: It is commonplace to refer to the Maldon Division as the ’political cock-pit of Essex,’ but it has a record far more significant than being the home for slap-dab hustings.
1956		Austin American (TX) 24 June 28/3: The very modern home features two [...] courtyards, one of them slab-dab in the center of the house.
1958		A. Buckeridge Jennings’ Little Hut 178: Fancy running slap-bang-doyng into Mr. Carter like that.
1975		Tampa Trib. (FL) 11 Oct. 16/1: The scoreboard [is] slab-dab in the middle of left centre field.
1984		Newcastle Eve. Chron. 20 Sept. 4/3: You are slapdab in the middle of it.
1990		Times Democrat (Orangeburg, SC) 15 Sept. 8/1: I came out of the hotel and ran slab-dab into lewis Stone.
1999		Indep. The Information 24–30 July 47: Blackadder hits the truth, but never slap-bang in the middle.

2. unmitigated, unrestrained.

1948

Essex Newsman 22 Oct. 3/6: It is commonplace to refer to the Maldon Division as the ’political cock-pit of Essex,’ but it has a record far more significant than being the home for slap-dab hustings.