Figuratively, "the smallest possible quantity," from late 14c. From early 15c. in English as the smallest unit of weight (originally the weight of a plump, dry grain of wheat or barley from the middle of the ear). From late 14c as "roughness of surface; a roughness as of grains." In reference to wood, "quality due to the character or arrangement of its fibers," 1560s; hence, against the grain (1650), a metaphor from carpentry: cutting across the fibers of the wood is more difficult than cutting along them.
Earliest sense of the word in English was "scarlet dye made from insects" (early 13c.), a sense also in the Old French collateral form graine; see kermes for the evolution of this sense, which was frequent in Middle English; also compare engrain. In Middle English grain also could mean "seed of flowers; pip of an apple, grape, etc.; a berry, legume, nut." Grain alcohol attested by 1854.
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, "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, "easily touched by emotion, readily wounded by unkindness" (but also sometimes "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 purely physical sense, in reference to a living being, skin, etc., "having quick or intense response to sensation," is by 1808. Of devices, mechanisms, etc., "so delicately adjusted as to respond quickly to very slight changes of condition," by 1857. The Cold War meaning "involving national security" is attested by 1953. Related: Sensitively; sensitiveness.
The older meaning 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).
Old English bean "bean, pea, legume," from Proto-Germanic *bauno (source also of Old Norse baun, Middle Dutch bone, Dutch boon, Old High German bona, German Bohne), and related to Latin faba "bean;" Greek phakos "lentil;" Albanian bathë "horse-bean;" Old Prussian babo, Russian bob "bean," but the original form is obscure. Watkins suggests a PIE reduplicated root *bha-bhā- "broad bean;" de Vaan writes that the Italic, Slavic and Germanic "are probably independent loanwords from a European substratum word of the form *bab- (or similar) 'bean'."
As a metaphor for "something of small value" it is attested from c. 1300 (hill of beans as something not much to amount to is from 1863). Meaning "head" is U.S. baseball slang 1905 (in bean-ball "a pitch thrown at the head"); thus slang verb bean meaning "to hit on the head," attested from 1910. Bean-shooter as a child's weapon for mischief, a sort of small sling-shot to fire beans, is attested from 1876. Slang bean-counter "accountant" recorded by 1971.
The notion of lucky or magic beans in English folklore is from the exotic beans or large seeds, carried from the Caribbean or South America by the Gulf Stream, that wash up occasionally in Cornwall and western Scotland. They were cherished, believed to ward off the evil eye and aid in childbirth.
To not know beans "be ignorant" is attested by 1842 in American English, often said to be a New England phrase; it is perhaps from the "object of little worth" sense. Some of the earliest citations give it in a fuller form, but they do not agree: "why, I sometimes think they don't know beans when the bag is open" ["The History of the Saints," 1842]; "This feller don't know beans from porridge, no how." ["Etchings of a Whaling Cruise," 1850]. It might have a connection to the English colloquial expression know how many beans make five (1824) "be a clever fellow."