The Collaborative International Dictionary of English v.0.48:

Informidable \In*for"mi*da*ble\ ([i^]n*f[^o]r"m[i^]*d[.a]*b'l),
   a. [L. informidabilis. See In- not, and Formidable.]
   Not formidable; not to be feared or dreaded. [Obs.] "Foe not
   informidable." --Milton.
   [1913 Webster]