Antonyms of adj plausible

2 senses of plausible

Sense 1
plausible (vs. implausible) -- (apparently reasonable and valid, and truthful; "a plausible excuse")

implausible (vs. plausible) -- (having a quality that provokes disbelief; "gave the teacher an implausible excuse")
        => improbable, unbelievable, unconvincing, unlikely -- (having a probability too low to inspire belief)

Sense 2
plausible -- (given to or characterized by presenting specious arguments; "a plausible liar")

INDIRECT (VIA insincere) -> sincere -- (open and genuine; not deceitful; "he was a good man, decent and sincere"; "felt sincere regret that they were leaving"; "sincere friendship")

Similarity of adj plausible

2 senses of plausible

Sense 1
plausible (vs. implausible) -- (apparently reasonable and valid, and truthful; "a plausible excuse")
       => arguable -- (capable of being supported by argument)
       => glib, pat, slick -- (having only superficial plausibility; "glib promises"; "a slick commercial")
          Also See-> credible#1, believable#1

Sense 2
plausible -- (given to or characterized by presenting specious arguments; "a plausible liar")
       => insincere (vs. sincere) -- (lacking sincerity; "a charming but thoroughly insincere woman"; "their praise was extravagant and insincere")