sensitivity n.

late 14c., sensitif, in reference to the body or its parts, "capable of receiving impressions from external objects, having the function of sensation;" also (c. 1400) in scholastic philosophy, "of or pertaining to the faculty of the soul that receives and analyzes sensory information;" from Old French sensitif "capable of feeling" (13c.) and directly from Medieval Latin sensitivus "capable of sensation," from Latin sensus, past participle of sentire "feel, perceive" (see sense (n.)). Also in early Modern English sencitive.

By 1520s as "of, connected with, or affecting the senses." With reference to persons or mental feelings, "keenly susceptible to external influences," especially "easily touched by emotion, readily wounded by unkindness" (but also "ready to take offense"), by 1816.

What is commonly called a 'sensitive' person is one whose sense-organs cannot go on responding as the stimulus increases in strength, but become fatigued. [James Sully, "Outlines of Psychology," 1884]

The mechanical meaning "so delicately adjusted as to respond quickly to very slight changes or conditions" is by 1857. The Cold War meaning "involving national security" is attested by 1953. Related: Sensitively; sensitiveness.

The purely physical sense, in reference to a living being, skin, etc., "having quick or intense response to sensation," is by 1808; it is preserved in sensitive plant (1630s, also in Shelley's poem), a legume which is "mechanically irritable in a higher degree than almost any other plant" [Century Dictionary]. 

Marijuana ... makes you sensitive. Courtesy has a great deal to do with being sensitive. Unfortunately marijuana makes you the kind of sensitive where you insist on everyone listening to the drum solo in Iron Butterfly's 'In-a-Gadda-Da-Vida' fifty or sixty times. [P.J. O'Rourke, "Modern Manners," 1983] 

As a noun, in mesmerism, "one who is sensitive to hypnotic influence," 1850; later "one in whom the sensitive facility is highly developed, an aesthete" (1891).

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]