As heard - like 'Hump' - on 'NYPD Blue'. I have a vague idea it means 'scumbag' (a phrase that has not really taken hold in the UK, I don't think).
Something I did hear in 'NYPD Blue' was a reference to a 'paper hanger' meaning someone who kites cheques (yes?). That phrase was around in Victorian England.
Still my favourite line: 'Ipso this, you pissy little bitch'.
Skell, hump, mook, mope -- all refer to petty criminals, lowlifes, etc. Pretty much interchangeable.
A paper hanger is indeed a check kiter -- a type of crime that is now largely obsolete, thanks to the marvels of modern technology.
'Mook' I knew from 'Mean Streets'. Thanks, Patrick, I thought I was on the right track.