circular, round ...1861 rare
a round thing ...1661 obs. rare
from rotund (adj. round, orbicular) + -ant
FIRST DOCUMENTED USE
1661 - see EXAMPLE below
"...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: Dictionaries only show 'rotundant' as being a noun.
However, there are examples of it being used as an adjective (circular, round), as shown in the following example.
"...a third lugged a looking glass under his arm, a fourth minced in Broadway style, enveloped in a rotundant hoop skirt, and so on, each of the squad carrying off a trophy..."
From: The Iowa First: Letters from the War
- Franc Bangs Wilkie, 1861