We don't have synonyms for unthrifty yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are thrifty, unthrift and unthriftily.
(adj) :
given to, or evincing, thrift; characterized by economy and good management of property; sparing; frugal; thriving by industry and frugality; prosperous in the acquisition of worldly goods; increasing in wealth; as, a thrifty farmer or mechanic
(adj) :
given to, or evincing, thrift; characterized by economy and good management of property; sparing; frugal; thriving by industry and frugality; prosperous in the acquisition of worldly goods; increasing in wealth; as, a thrifty farmer or mechanic