mid-15c., "intense, fervent, great," from Old French intensif (14c.) and Medieval Latin intensivus, from Latin intens-, past participle stem of intendere "turn one's attention; strain, stretch" (see intend).
Grammatical meaning "expressing intensity" is from c. 1600; as a noun, "something expressing intensity," 1813, from the adjective. Alternative intensitive is a malformation. Intensive care attested from 1958. Related: Intensively; intensiveness.