look up any word, like half chub:
 
1.
Receiving a blowjob whilst taking a shit
Eg 1.

Guy : *drops a brown one* oh yeah! *looks down* OH YEAH!!!

Girl : Why the hell am i giving this guy a plumby!?

Eg 2.

Guy 1 : So, i heard you problems with the toilet last night

Guy 2 : Nah, me and the Missus sorted it out, in the end she gave me a plumby and we didn't bother to repair the leak
by plumbysmasher November 13, 2009
 
2.
A legendry figure from the Mabinogion, an assembly of Welsh Storys. Plumby is said to have killed 100's or English Soldiers during Wales' struggle for independance around 500BC. Plumby used his large testicles to crush the soldiers.
You should have had a better saddle on your bike, your walking like Plumby.
by Timothy Spall September 20, 2006