-ern

In form, from Middle English -erne, -ern, -ren, -ron (northern(e), northron, so(u)thern(e), sothron, etc), from Old English -erne (norþerne, etc), from Proto-Germanic *-r- (probably from rebracketing of *nurþrōnijaz etc) + *-ōnijaz, whence also Old High German -rōni, Old Saxon -rōni, Old Norse -rǿnn / Old Norse -ǿnn. In practice, possibly a back-formation from northern, southern, etc.
(Contrast the -ern in hāliġern, etc., which is related to ærn (“place”).)