Antonyms of adj well-founded

1 sense of well-founded

Sense 1
tenable, well-founded -- (based on sound reasoning or evidence; "well-founded suspicions")

INDIRECT (VIA reasonable) -> unreasonable -- (not reasonable; not showing good judgment)

Similarity of adj well-founded

1 sense of well-founded

Sense 1
tenable, well-founded -- (based on sound reasoning or evidence; "well-founded suspicions")
       => reasonable (vs. unreasonable), sensible -- (showing reason or sound judgment; "a sensible choice"; "a sensible person")