E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
in sport, means that the gun was discharged incessantly; it went slap here and bang there. As a term of laudation it means very dashing, both words being playful synonyms of dashing, the repetition being employed to give intensity. Slap-bang, here we are again, means, we have popped in again without ceremony. Pop, slap, bang, and dash are interchangeable.
Dickens uses the word to signify a low eating-house.
They lived in the same street, walked to town every morning at the same hour, dined at the same slap-bang every day.