
With the whole of Google at your fingertips you need me to explain.
I discovered the word from the
Words column in
the Oldie magazine. Originally mumchance was a game (cards I think but I'm not going looking it all up again) that had to be played in silence, from this a mumchancer came to be someone who looked on without contributing, it can be either from a point of advantage or because one hasn't the bottle, it is in the latter meaning that HG Wells uses it in his
'The Plattner Story'. I thought it suitable, and much more entertaining, as a description of those others call lurkers - those who join forum and don't post.
From the same article came rantipole, someone who acts childishly by doing such acts as kicking the furniture in frustration and giving vent to rants about things, rantipole is a self-conferred title that I carry from my days on the old Tiscali forum (and others) where my occasional outbursts of fury and malarkey would give entertainment (or not) to other forum users.
As rob has pointed out, either because he is a member there who I don't recognise or because his Googling skills are up to scratch, I am a current member of a small social site called
Salem's Lot where I have been known to use the term mumchancer, and where I carry the title rantipole. Googling mumchancer will also give many of the other sites where I am/have been a member. Despite my efforts to get it adopted all over the web, it seems I am still the only user of the term.

Currently investigating the Hillmans of Sussex.