Etymology

The sign has flopped down, but the eponymous farm is up on the hill above us.

'Plan' refers to this hill, a patois variation of the Occitan 'pech'.
'Coulon' from 'Couillon'(idiot) which comes from the French 'couille' (a bollock/testicle)

Thus we end up with Testicle Hill: the Lafage version of 'The Fool on the Hill'