unconvincing (adj.)

• absurd
• doubtable
• doubtful
• dubious
• dubitable
• flimsy
• implausible
• improbable
• inconceivable
• incredible
• preposterous
• problematic
• questionable
• ridiculous
• suspect
• suspicious
• tall
• thick
• thin
• unbelievable
• unearthly
• ungodly
• unimaginable
• unsubstantial
• unthinkable
• weak