well-grounded

well-grounded (comparative better-grounded or more well-grounded, superlative best-grounded or most well-grounded)

1. Of a person: having extensive knowledge of a subject.

   Synonym: (archaic, rare) groundly

2. Of an argument, etc.: based on good judgment and solid reasoning.

   Synonyms: well-founded, on solid ground

   Antonyms: ill-grounded, ill-founded, groundless, baseless, flimsy, unjustified