E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Slap-up.
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.”
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“[The] more slap-up still have the shields painted on the panels with the coronet over.”—Thackeray.
