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1670s, from slap (v.) + dash (v.). As an adjective, "dashing, offhand, careless," from 1792. As a noun, "rough-coat, coarse plaster," from 1796.
Precisely; directly: Streets that ended slap in a courtyard/ The storm was pointed slam-bang at Tampa (first form 1829+, second 1885+, third 1940s+)