The Menapii occupied a large territory stretching along the coast from the Aa to the mouth of the Maas. It seems to have been thinly populated and Caesar tells us that in the first century B.C.E. it was full of marshes and forests. Inland their frontier lay on the upper Lys, the Deule and the Schelde. By the first century C.E., their principle city under Roman administration was Castellum Menapiorum (Cassel). Their territory seems to have been reduced and centred further south than Caesar reported. Land seems to have been taken from the Morini east of the Aa. This may have been linked to the disturbances on the lower Rhine caused by the Suebi in 30-29 B.C.E. and used by the Morini as an occaision for revolt.

A branch of the Menapii is recorded by Ptolemy as a tribal name "Manapioi" in Ireland. The form of this name recorded by Ptolemy in the second century C.E., with the first "e" pronounced "a" in sympathy with the second vowel is evidence that they are more recent than the continental Menapii.

The etymology of their name is obscure. Some linguists propose a non-Celtic word, later Celticized, as the origin.