E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Prime slap-up or slap-bang-up. Very exquisite or dashing. Here slap is a playful synonym of dashing, and up is the Latin super, as in superfine. The dress of a dandy or the equipage of an exquisite is slap-up, prime slap-up, or slap-bang-up.
[The] more slap-up still have the shields painted on the panels with the coronet over.Thackeray.