Sobiest’s post on peculation led me to wonder, after all these years, why Latin nouns and verbs are called first/second/… etc declensions/conjugations? Who decided this and when?

I’ve got a copy of a 1709 grammar that numbers the conjugations of the verbs, so it was sometime before that.

At least since Priscianus’ grammar (6th century CE). (While looking for an online copy, I ran across this great resource (link), The Corpus grammaticorum Latinorum.)

Ah, dear old Priscian! To break Priscian’s head is an old expression meaning to make grammatical errors.

Some, free from rhyme or reason, rule or check,
Break Priscian’s head and Pegasus’ neck.

Aelius Donatus, 4th century AD, was the go-to guy for the Middle Ages and beyond when it came to Latin Grammar.

Edited to delete superfluous link (jheem got there before me!)