'Slap up' meant well-to-do types in the 1800s, so it tended to mean good class, high quality, first rate or stylish. Couldn't give definitive who used it first though. Like most expressions, it seems to have evolved from general language, like English is still doing now.
'Slap-up' is a more recent variation of 'bang-up', meaning 'excellent'. The word' 'bang' here was used in the same way as it is in the phrase 'bang in the middle'. This suggests it is exactly there or exactly right. Thus, bang-up and later slap-up mean 'just the ticket' or 'the very thing' etc...ie a wonderful meal, for example.
As regards the origin, which I forgot to mention in my earlier response, it first appeared in a book about slang published in 1823. There, it simply says it is a northern version of 'bang-up', as I outlined above.