from rotund (adj.) + -ant, after quadrant
“…He is a good anatomist to scrue into the very center of a loaf, and to pry into the joynt of separation. A good surveyour only, he measures not by the chaine nor the quadrant, no, by the retundant* rather, i.e. the jugg…”
From: Confused Characters of Conceited Coxcombs,
Or, A Dish of Traitorous Tyrants
Note: – * ‘retundant’ as shown in the above example is correct
– The Oxford English Dictionary only shows ‘rotundant’ as a noun.
However, there are examples of it being used as an adjective, as in this example from 1846:
“…“Oh!” exclaimed the rotundant figure of the queen…”