Synonyms of well-grounded

2 Syllables
valid, cogent, learned, versed
3 Syllables
pansophic, silhouetted, polymath, tangible
4 Syllables
suasive, definite, incarnated, professorial, undistorted, philosophic, substantiated
5 Syllables
undubitable, corroborated, well-educated, unquestionable