late 14c., in reference to the body or its parts, "having the function of sensation;" also (early 15c.) "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.)).
Meaning "easily affected" (with reference to mental feelings) first recorded 1816; meaning "having intense physical sensation" is from 1849. Original meaning is preserved in sensitive plant (1630s), which is "mechanically irritable in a higher degree than almost any other plant" [Century Dictionary]. Meaning "involving national security" is recorded from 1953. Related: Sensitively; sensitiveness.