Green’s Dictionary of Slang

a general intensifier, completely, entirely, absolutely, quite.

1802		Sporting Mag. Nov. XXI 103/1: Plunging and prancing [...] / Comes spank thro’ a hedge with a thundering crush.
1831		Satirist (London) 22 May 56/3: ‘I wonder [...] will the gentleman catch that vile rascal wot hit him so spank’.
1842		‘Nancy Dawson’ in Nancy Dawson’s Cabinet of Songs 10: Then cried my lads, come fire away, / Spank into Nancy Dawson.
1855		(con. 1852) Fights for the Championship 248: Broome let go his left spank in Orme’s mouth.
1866		Wild Boys of London I 7/1: He hits him spank in the jaw.
1902		Bulletin (Sydney) 5 Jul. 36/2: He must’ve thought it was home, seein’ ladies outside. I’d got pretty close up – when down goes Nugget. Turned me a turtle spank among the petticoats!
1956		C. Knight We Were There at the Normandy Invasion 77: Without a doubt, those Nazi officers have run spank into the manure pile.
1962		(con. 1940s) G. Mandel Wax Boom 15: I got to take him by the hand today, gonna drive him spank into our own junk for his first kiss.