from rotund adj., after quadrant
Note: E-NED only shows 'rotundant' as being a noun. However, there are examples of it being used as an adjective, as shown in the second example.
Also note that the first example shows the spelling as 'retundant,' as per E-NED.
From: Confused Characters of Conceited Coxcombs,
Or, A Dish of Traitorous Tyrants
Edited by James O. Halliwell, 1860
A Colledge Butler
From: Miscellanies in Prose and Verse
By Benjamin Hardacre, 1874
I Would Not Be a Bird