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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.