Words related to *ak-
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 ("sour, sharp, biting") is from 1775; the word was applied to intense colors by 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."
"highest point," 1560s, from Greek akmē "(highest) point, edge; peak of anything," hence "prime (of life, etc.), the best time" (from PIE *ak-ma-, suffixed form of root *ak- "be sharp, rise (out) to a point, pierce"). In English it was written in Greek letters until c. 1620. The U.S. grocery store chain was founded 1891 in Philadelphia, Pennsylvania.
1712, "sharp and bitter to the taste," formed irregularly (perhaps by influence of acrimonious) from Latin acer (fem. acris) "sharp to the senses, pungent, bitter; eager, fierce," also figuratively, of qualities, "active, ardent, spirited," also "hasty, quick, passionate;" of mind "violent, vehement; subtle, penetrating," from PIE *akri- "sharp," from root *ak- "be sharp, rise (out) to a point, pierce." Of feelings, temper, etc., in English from 1781. The -id suffix probably is in imitation of acid. Acrious (1670s) is a correct formation, but seldom seen. Related: Acridly.
1540s, "quality of being sharp or pungent in taste," from French acrimonie or directly from Latin acrimonia "sharpness, pungency of taste," figuratively "acrimony, severity, energy," abstract noun from acer "sharp" (fem. acris), from PIE root *ak- "be sharp, rise (out) to a point, pierce") + -monia suffix of action, state, condition (see -mony). Figurative extension to personal sharpness or bitterness is by 1610s.
word-forming element meaning "highest, topmost, at the extremities," before vowels acr-, from Latinized form of Greek akro- "pertaining to an end, extreme," from akros "at the end, at the top, outermost; consummate, excellent" (from PIE *akri-, from root *ak- "be sharp, rise (out) to a point, pierce").
1845, from French acrobate "tightrope-walker" (14c.) and directly from a Latinized form of Greek akrobatēs "rope dancer, gymnastic performer," which is related to akrobatos "going on tip-toe, climbing up high," from akros "topmost, at the point end" (from PIE root *ak- "be sharp, rise (out) to a point, pierce") + Greek agential element -batēs "one that goes, one that treads (in some manner), one that is based," from -batos, verbal adjective from stem of bainein "to go, walk, step" (from PIE root *gwa- "to go, come").