''To have a slap up meal means to eat well. The expression goes back to the time of Charles Dickens, when it was a "slap-bang" meal, derived from cheap eating houses, where one slapped one's money down as the food was banged on the table. Why "down" has turned to "up" is probably another example of language evolution.''
used to be a meal at a cheap "slap-bang" restaurant: you slapped your money on the table as they banged the food down in from of you. It's sort of mutated into slap-up and you may sometimes hear bang-up meal too, though that's well out of fashion these days.
Excuse me you lot. You are all great and I'd like to treat you all to a slap-up (or slap-down) meal, but BOO and I go back a long way, so sorry O.G, I'm turning you down for a faded poppy - and your treatment looks more like Dead Sea mud to me
What about the Ritz, BOO ?
Luv yer ! xxx
Oh, and thanks for the informative answers the rest of you.