unweighty

un- +‎ weighty. Attested since the 17th century.

unweighty (comparative more unweighty, superlative most unweighty)

1. Not weighty.
   • 1676, Edward Earl of Clarendon, A brief view and survey of the dangerous and pernicious errors to Church and State in Mr Hobbes's Book entitled Leviathan:

     […] the instances and arguments given by him are very unweighty and trivial to conclude the nature of man to be so full of jealousie and malignity, as he would have it believed to be, from that common practice of circumspection and providence, which custom and discretion hath introduced in human life.

• unconvincing
• unpersuasive