The Collaborative International Dictionary
Improsperous
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.]
Wiktionary
improsperous
a. (context obsolete English) Not prosperous.