The Collaborative International Dictionary of English v.0.48:
Improsperous \Im*pros"per*ous\, a. [Pref. im- not + prosperous: cf. F. improsp[`e]re, L. improsper.] Not prosperous. [Obs.] --Dryden. -- Im*pros"per*ous*ly, adv. [Obs.] -- Im*pros"per*ous*ness, n. [Obs.] [1913 Webster]