a Gaulish word Latinized

Probably. Also, as with some many other Celtic tribes, there was more than one tribe known as the Parisii, one in Gallia and the other in Britannia, in modern-day East Riding of Yorkshire. The usual etymology is that it is related to Old Irish cuirim 'I place, lay, put' + a suffix -isio. The German philologist and founder of modern Cletic language studies Johann Kaspar Zeuß glosses Parisii as 'efficaces, strenui' and 'Tatkräftigen, Tapferen' ('the vigorous, brave, doughty', link). He posited that it was from an earlier Quarisii, the change of qu to p being a common one in Celtic languages.

Ceci n'est pas un seing.