"to pay out or away" (money or wealth), Old English -spendan (in forspendan "use up"), from Medieval Latin spendere, a shortening of Latin expendere "to weigh out money, pay down" (see expend) or possibly of dispendere "to pay out." A general Germanic borrowing (Old High German spendon, German and Middle Dutch spenden, Old Norse spenna). In reference to labor, thoughts, time, etc., attested from c. 1300. Intransitive sense "exhaust, wear (oneself) out" is from 1590s (see spent).
c. 1300, "fact or condition of thriving," also "prosperity, savings," from Middle English thriven "to thrive" (see thrive), influenced by (or from) Old Norse þrift, variant of þrif "prosperity," from þrifask "to thrive." Sense of "habit of saving, economy" first recorded 1550s (thrifty in this sense is recorded from 1520s; also see spendthrift). Thrift shop attested by 1919.