The maquis is an untempting semi-arid scrubland made up of thorny, sharp, un-nutritious and inhospitable vegetation. It spreads out over a substantial part of the duchy of Quirm, and as luck would have it, is right on the best, in fact the only, route for the railway to take from Ankh-Morpork. Only the marginalised and desperate live there, and those who would fight a guerilla war against ever-encroaching authority. Moist von Lipwig and various associate enter the Maquis to negotiate with one and drive out the other.
The Maquis country covers a large amount of the Massif Central in France and passes through the south to the island of Corsica, which is all Maquis. During WW2 it became a haven and a pretty much safe base for the various sorts of French Resistance groups, who would periodically launch a raid from this safe base on their sworn enemies. (and after they'd finished fighting the Communist/Anarchist/deGaulliste/pro-British/pro-American/pro-Soviet Resistance, they might even ambush the Germans). This other sort of maquis was called a maquisard.