What is the etymology of 'slap-bang'.?

Solution 1:

It is probably a variant of the apparently older and similar expression smack bang:

The OED points out that the verb "smack" can be used as an adverb to mean "With, or as with, a smack; suddenly and violently; slap. Also with down, through, etc.

The quotations given to illustrate this sense are:

1782 COWPER Smack went the whip, round went the wheels. 1799 GEO. [IV] in Paget Papers He..tumbled..smack on his face. 1806 H. SIDDONS Smack comes a ball from the enemy and carries away his head. 1836 T. HOOK So away I went smack bang into a quaker's shop to buy myself a pair of gloves.

As you can see, by 1836 "smack-bang" already existed, doubling the effect of "smack" on its own. (VSD)

The OED shows also a further development of the adverbial sense, to mean "completely, entirely; directly," and this is the way I usually hear it used - for instance, to be "smack in the middle of something," or in the quotations cited:

1828 WHEWELL We have got a decision which is smack against us. 1857 A. MATHEWS The wind being smack in their teeth the greater part of the voyage. 1864 TYTLER [Cardan] made the bishop smack whole in twenty-four hours. And these too can be doubled for emphasis, "smack-bang." - Baceseras.

(Phrase Finder)