late 14c., prioritie, "state of being earlier (than something else), prior occurrence or existence," from Old French priorite (14c.) and directly from Medieval Latin prioritatem (nominative prioritas) "fact or condition of being prior" (source also of Spanish prioridad), from Latin prior (see prior (adj.)).
From c. 1400 as "precedence in right, place, or rank." In 1897 Century Dictionary wrote "Priority is the state or fact of coming first in order of time; what little use it has beyond this meaning is only a figurative extension," but in 20c. the sense shifted toward "fact or condition of coming first in importance or requiring immediate attention; thing regarded as more important than another or others." Wyclif (early 15c.) renders prioritas into (Middle) English as furtherhead.