1817 (transitive), from intense + -ify, first attested in Coleridge, in place of intend, which he said no longer was felt as connected with intense. Intransitive sense is from 1845. Middle English used intensen (v.) "to increase (something), strengthen, intensify," early 15c. Related: Intensified; intensifying.
I am aware that this word [intensifying] occurs neither in Johnson's Dictionary nor in any classical writer. But the word, "to intend," which Newton and others before him employ in this sense, is now so completely appropriated to another meaning, that I could not use it without ambiguity: while to paraphrase the sense, as by render intense, would often break up the sentence and destroy that harmony of the position of the words with the logical position of the thoughts, which is a beauty in all composition, and more especially desirable in a close philosophical investigation. I have therefore hazarded the word, intensify; though, I confess, it sounds uncouth to my own ear. [Coleridge, footnote in "Biographia Literaria," 1817]