1530s, "making clear or manifest, explanatory," from Middle French déclaratif and directly from Late Latin declarativus, from past-participle stem of Latin declarare "make clear, reveal, disclose, announce," from de-, here probably an intensive prefix (see de-) + clarare "clarify," from clarus "clear" (see clear (adj.)).
Meaning "making declaration, exhibiting" is from 1620s. The word was used in mid-15c. as a noun meaning "an explanation." Related: Declaratively.