In Dickens' David Copperfield, there is an exchange between David and Mr Pegotty who arrives with his nephew Ham to visit him at school. It runs as follows:
"Do you know how mama is, Mr Peggotty? I said. And how my dear, dear old Peggotty is?
'Oncommon' said Mr Peggotty.
'And little Em'ly, and Mrs Gummidge?'
'On--common', said Mr Peggotty.
And following a further exchange Peggotty continues:
…I was to come over and inquire for Mas'r Davy and give her dooty, humbly wishing him well and reporting of the fam'ly as they was oncommon toe-be-sure. Little Em'ly, you see, she'll write to my sister when I go back, as I see you and as you was similarly oncommon, and so we make it quite a merry-go-rounder.
Clearly oncommon means something like "in the best of health" but not only is it not quoted in the OED - even as an historic archaism - but as one who grew up close to the Norfolk dialect, which Peggotty speaks, it is unfamiliar to me too.