a. 无法使人信服的, 难以说服人的

Inconvincible \In`con*vin"ci*ble\, a. [L. inconvincibilis. See
{In-} not, and {Convince}.]
Not convincible; incapable of being convinced.

None are so inconvincible as your half-witted people.
--Gov. of the
Tongue.