The Collaborative International Dictionary of English v.0.48:

Insolidity \In`so*lid"i*ty\, n. [Pref. in- not + solidity: cf.
   F. insolidit['e].]
   Lack of solidity; weakness; as, the insolidity of an
   argument. [R.] --Dr. H. More.
   [1913 Webster]