1540s, from Latin Pythagoreus "of or pertaining to Pythagoras" of Samos, Greek philosopher (6c. B.C.E.) said to have travelled to Egypt and Babylon, whose teachings included transmigration of the soul and vegetarianism (these are some of the commonest early allusions in English).
Also in reference to a school he supposedly founded in Crotona in Italy. As a noun, "a follower of Pythagoras," by 1540s. The Pythagorean theorem is the 47th of the first book of Euclid: The area of the square whose side is the hypotenuse of a right triangle is equal to the sum of the areas of the squares on the other two sides.