Also formerly used for the P-Celtic languages, that is the Celtic languages of Wales, Brittany, and Cornwall. William Petty in his 1691 book The Political Anatomy of Ireland, compares Manx to what he calls the Cimbric languages:
The tongue now spoken in Wales, and in Basse
Bretagne, and which was formerly spoken in Cornwall,
differs from it essentially. By the latter tribes, their
native tongue is called Cimbric. It must be admitted,
however, that in all these dialects, strong marks of
similarity occur, as well in the words themselves, as in
the idioms and grammatical construction of each dialect.
In particular, the inflexions of their nouns and verbs
are effected each by changes of the initial consonant,
according to fixed rules. In one respect the Manks
approaches more nearly to the Cimbric than to its
sister-dialects spoken in Ireland and the Highlands.
The Bas Breton, and the Welsh, appear, on some
occasions, to employ a dual number, as does the Manks,
and as did formerly the extinct Cornish.
The name is of course related to the Welsh name Cymry, meaning Wales, and to the county of Cumberland and the Cambrian geological period (named from Welsh rock strata).
Pinkerton in his Enquiry into the History of Scotland (1814) insists the Cymry of Britain were Cimbrii migrated from Germany (or the "Cimbric Chersonese" in Denmark), the same people being also known to the Greeks as Kimmerioi (the phrase Cimmerian darkness being proverbial), dwelling north of the Black Sea,and even known to the Assyrians as Gimmirrai.
Roman history describes a Cimbric attack on Rome. They call them Germanic, but this may be just a place name designation (as they came from Denmark it's close enough), and fairly obviously they were actually part of the common Celtic culture of Europe. The Gundestrup Cauldron was obtained as booty in this raid. | 
