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[...The people which in history is called Prussian is the population that in the migration of nations settled in that part of the Baltic coast-land which in the second half of the Middle Ages was known as Prussia. Their name Pruzi, or, in its lengthened form, Prutheni (their country, Prucia or Prussia), is derived from the Lithuanian Protas, i.e., in sight, understanding: they called themselves Pruzi, the sagacious....]

the above text is taken from the following website:
http://www.ccel.org/s/schaff/encyc/encyc09/htm/iv.v.lxi.htm

The surname Protas means probably 'the wise' (as a nickname), it anyhow refers to wisdom as the above text showed, but 'the wise' would be the most logical translation as many languages have such surnames, e.g. English = Wise; Dutch = De Wijs; German = Weise, Klug; et cetera.