Etymology of the word "normal" (perpendicular)

Solution 1:

Latin normalis stems from (latin) norma - a tool with a right angle, a carpenter's square.

Solution 2:

From the OED Third Edition, an entry updated December 2003:

Etymology: < classical Latin normālis right-angled, in post-classical Latin also conforming to or governed by a rule (4th–5th cent.) < norma + -ālis. Compare French normal (1450–65 in Middle French in an isolated attestation in verbe normal (compare sense A. 1), then from mid 18th cent., earliest in ligne normale (1753; compare sense A. 5), and subsequently in more general senses ‘which serves as a model’ (1793 in école normale, 1803 in more general use), ‘ordinary, regular’ (1830s); compare earlier anormal adj.), Italian normale according to the norm, routine, predictable, common, boring (1683 in sense ‘perpendicular, orthogonal’, 1831 in sense ‘customary, expected’), Portuguese normal (1844), Spanish normal (1855).

In short, it goes back to the Latin source of the word.