1660s, from intense + -ity. Earlier was intenseness (1610s). A scientific term originally; sense of "extreme depth of feeling" attested by 1830.
Entries linking to intensity
intense adj.
early 15c., of situations or qualities, "great, extreme," from Old French intense (13c.), from Latin intensus "stretched, strained, high-strung, tight," originally past participle of intendere in its literal sense of "stretch out, strain" (see intend). From 1630s of persons, "high-strung." Related: Intensely; intenseness.
-ity
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]