early 14c., "plane figure having four equal sides and two acute and two obtuse angles," from Old French losenge "rhombus shape, diamond-shape" (as an ornamental motif in heraldry, etc.); "small square cake; windowpane," etc., a word used for many flat quadrilateral things (Modern French losange). It has cognates in Spanish losange, Catalan llosange, Italian lozanga, but the origin is disputed.
Probably from a pre-Roman Celtic language, perhaps Iberian *lausa or Gaulish *lausa "flat stone" (compare Provençal lausa, Spanish losa, Catalan llosa, Portuguese lousa "slab, tombstone"). From late 14c. as "diamond-shaped cake or wafer;" specific sense "small cake or tablet (originally diamond-shaped) of medicine and sugar, etc., meant to be held in the mouth and dissolved" is from 1520s.
The related words in Continental languages often have a sense "flattery, deceit" (compare Old French losengier "to praise unduly," losenge "flattery, false praise; deceitful friendliness"), which comes probably via the notion of square flat slabs of tombstones and their fulsome epithets. Some of this made its way into Middle English via French. Chaucer uses losenger "flatterer, deceiver;" losengerye "flattery."
geometric figure, "oblique-angled equilateral parallelogram," 1570s, from French rhombe, from Latin rhombus "a magician's circle," also a kind of fish, which in Late Latin took on also the geometric sense. This is from Greek rhombos "circular movement, spinning motion; spinning-top; magic wheel used by sorcerers; tambourine;" also "a geometrical rhomb," also the name of a flatfish.
Watkins has this from rhembesthai "to spin, whirl," from PIE *wrembh-, from *werbh- "to turn, twist, bend" (source also of Old English weorpan "to throw away"), from root *wer- (2) "to turn, bend" (see versus). But Beekes connects rhombos to rhembomai "to go about, wander, roam about, act random," despite this being attested "much later," a word of no clear etymology.
In general use in reference to any lozenge-shaped object. Related: Rhombic.
Middle English drope, from Old English dropa "a small, spherical mass of liquid," from Proto-Germanic *drupon (source also of Old Saxon dropo, Old Norse dropi, Dutch drop, Old High German tropfo, German Tropfen (n.)); see drop (v.).
Sense of "minute quantity of anything, least possible amount" is from c. 1200. Meaning "an act of dropping" is from 1630s; of immaterial things (prices, temperatures, etc.) from mid-19c. Meaning "lozenge, hard candy" is 1723, from resemblance in shape. Meaning "secret place where things can be left illicitly and picked up later" is from 1931. Theatrical meaning "painted curtain dropped between scenes to conceal the stage from the audience" is by 1779.
Drop in the bucket (late 14c.) is from Isaiah xl.15 [KJV]. At the drop of a hat "suddenly" is from 1854. To get the drop on "be prepared before one's antagonist" originally was Old West gunslinger slang (1869).
1620s, "of the taste of vinegar," from French acide (16c.) or directly from Latin acidus "sour, sharp, tart" (also figurative, "disagreeable," etc.), adjective of state from acere "to be sour, be sharp" (from PIE root *ak- "be sharp, rise (out) to a point, pierce").
Figurative use in English is from 1775; applied to intense colors from 1916; an acid dye (1888) involves an acid bath. Acid rain "highly acidity in rain caused by atmospheric pollution" is recorded by 1859 in reference to England. Acid drop as a kind of hard sugar candy flavored with tartaric acid is by 1835, with drop (n.) in the "lozenge" sense. Acid test is American English, 1881, in literal use a quick way to distinguish gold from similar metals by application of nitric acid. Fowler wrote (1920) that it was then in vogue in the figurative sense and "became familiar through a conspicuous use of it during the war by President Wilson."