Apparently this is a peculiarly British term, but we'll sometimes use the phrase 'slap-up' to mean 'excellent', as in:
That's a slap-up meal!
They held a slap-up do.
What's the origin of the term slap-up? Nothing obvious comes to mind.
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The earliest meaning of slap up was to eat. Later it also meant to eat or drink in a hurry, and around the same time meant something exceedingly good.
Could the meaning transferred because a very good meal might be eaten quickly?
The 1898 Wit, character, folklore & customs of the North Riding of Yorkshire; with a glossary of over 4,000 words and idioms now in use defines it:
The earlier 1828 Dialect of Craven, in the westriding of the County of York has the former meaning:
This meaning to swallow is found in a 1709 French-English dictionary, a 1673 dictionary has it meaning to sustain, to support (possibly also meaning to eat) and in 1660 to lick. The swallow, eat meaning is printed in several English-French, -Danish, -Swedish, -Italian and Welsh dictionaries.
The exceedingly good meaning can be found in two 1823 slang dictionaries. Classical dictionary of the vulgar tongue by Francis Grose:
And second, in Slang by John Badcock, where bang-up is defined "quite in fashion, at the top of the mode. All right.":
The first non-dictionary use describes an exceptional boxing match in an 1821 Sporting Magazine:
The next use in prose I found is 1828's How to live in London:
gave 250 guineas for what he terms his "slap-up turnout," (i.e. handsome cabriolet, with a blood chesnut mare, 15 hands 3 inches, harness, &c.)
Some slightly later books apply slap-up to describe someone's fine clothes.
Both Partridge and Green have "slap-up" meaning "first-rate, of superior quality". The earliest reference they cite is from 1829, a little early for Dickens. The "slap-bang shop", a restaurant where one paid in advance, is earlier still (1796), cited in the Lexicon Balatronicum of 1811.
I would think that Green's earliest reference to "slap", the proceeds of a robbery, is the source of the phrase; that first appeared in 1790, apparently.