*normalis* already meant *right-angled* in classical Latin; for example, *angulus normalis* appears in the first century text *De institutione oratoria* (volume XI, paragraph 3.141) by <A HREF="https://en.wikipedia.org/wiki/Quintilian">Marcus Fabius Quintilianus.</A> 

In a <A HREF="http://catalog.hathitrust.org/Record/009268024">commentary</A> on this text from the fifteenth century this early use of the word "normalis" is explained as "rectus", see screenshot:

<IMG SRC="http://ilorentz.org/beenakker/MO/normalis.png">

"Angulus normalis est idem qui angulus rectus" = "a normal angle is the same as a right angle"

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In response to Ketil Tveiten's question: "How did normal come to mean ordinary" : according to this <A HREF="http://www.etymonline.com/index.php?term=normal">source,</A> the meaning of *normal* as *conforming to common standards* seems to be of recent origin (1828?).