insensitive adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

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).

1510s, "lacking or deprived of physical senses," from Late Latin insensatus "irrational, foolish," from in- "not" (see in- (1)) + sensatus "gifted with sense" (see sensate).

Meaning "irrational, maniacal, lacking or deprived of mental sense" is from 1520s; meaning "lacking or deprived of moral sense, unfeeling" is from 1550s. Insensate means "not capable of feeling sensation," often "inanimate;" insensible means "lacking the power to feel with the senses," hence, often, "unconscious;" insensitive means "having little or no reaction to what is perceived by one's senses," often "tactless." Related: Insensately; insensateness.