Green’s Dictionary of Slang

fellow commoner n.

[orig. Cambridge Univ. use, as opposed to scholars, commoners were ‘not in general considered as over-full of learning’ (Grose, 1785)]

an empty bottle.

[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Fellow commoner, an empty bottle, so called at the University of Cambridge, where fellow commoners are not in general considered as over full of learning; at Oxford an empty bottle is called a gentleman commoner for the same reason.
[UK]Gent.’s Mag. 1084: One [student at Cambridge] was a Harry Soph; another a fellow-commoner and senior Soph, and occasionally jocularly called an empty bottle, whilst e contra, a bottle decanted was, from time to time, denominated a fellow-commoner.
[UK]Lex. Balatronicum [as cit. 1785].
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.
[UK]Hotten Sl. Dict.
[UK]Sl. Dict.