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.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894