bean bag n.

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). The 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. Derisive slang bean-counter "accountant" is 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 "be a clever fellow" (1824).

"small sack," c. 1200, bagge, probably from Old Norse baggi "pack, bundle," or a similar Scandinavian source. OED rejects connection to other Germanic words for "bellows, belly" as without evidence and finds a Celtic origin untenable. In some senses perhaps from Old French bague, which is also from Germanic.

As disparaging slang for "woman" it dates from 1924 in modern use (but various specialized senses of this are much older, and compare baggage). The meaning "person's area of interest or expertise" is by 1964, from African-American vernacular, from jazz sense of "category," probably via notion of putting something in a bag. The meaning "fold of loose skin under the eye" is by 1867. Related: bags.

To be left holding the bag (and presumably nothing else), "cheated, swindled" is attested by 1793. Many figurative senses, such as the verb meaning "to kill game" (1814) and its colloquial extension to "catch, seize, steal" (1818) are from the notion of the game bag (late 15c.) into which the product of the hunt was placed. This also probably explains modern slang in the bag "assured, certain" (1922, American English).

To let the cat out of the bag "reveal the secret" is from 1760. The source is probably the French expression Acheter chat en poche "buy a cat in a bag," which is attested in 18c. French and explained in Bailey's "Universal Etymological English Dictionary" (1736), under the entry for To buy a pig in a poke as "to buy a Thing without looking at it, or enquiring into the Value of it." (Similar expressions are found in Italian and German; and in English, Wycliffe (late 14c.) has To bye a catte in þo sakke is bot litel charge). Thus to let the cat out of the bag would be to inadvertently reveal the hidden truth of a matter one is attempting to pass off as something better or different, which is in line with the earliest uses in English.

Sir Joseph letteth the cat out of the bag, and sheweth principles inimical to the cause of true philosophy, by wishing to make great men Fellows, instead of wise men ["Peter Pindar," "Peter's Prophecy," 1788]