normalnormal

The story

The earliest meaning of the word in English was "perpendicular; forming a right angle" like something normālis (‘made according to a carpenter's square’), but by Late Latin normālis had also come to mean "according to a rule", from which modern English senses of the word derive: in the 1800s, as people began to quantitatively study things like height, weight and blood pressure, the usual or most common values came to be called "normal", and by extension values regarded as healthy or desirable came to be called "normal" regardless of their usuality.

Full etymology on Wiktionary