Etymology
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alternating (adj.)

1550s, "occurring or acting by turns, one after the other," present-participle adjective from alternate (v.). Electrical alternating current is recorded from 1839, an electrical current which flows alternately in opposite directions without interruption.

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alternate (v.)
1590s, "do by turns" (transitive), from Latin alternatus, past participle of alternare "do one thing and then another, do by turns," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter). Replaced Middle English alternen "to vary, alternate" (early 15c.). Transitive meaning "interchange reciprocally" is from 1850; intransitive sense "follow one another in time or place" is from c. 1700; that of "pass back and forth between actions, conditions, etc." is by 1823. Related: Alternated; alternating.
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amoebaean (adj.)
also amoebean, "alternating, answering alternately," 1650s, from Greek amoibe "change, alteration; exchange" (see amoeba) + -an.
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AC/DC (adj.)
electronics abbreviation of alternating current/direct current, by 1898. As slang for "bisexual," 1959, said to have been in use orally from c. 1940; the notion is of working both ways.
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checker-board (n.)

also checkerboard, "board divided into 64 small squares of alternating color," 1779, from checker (n.1) + board (n.1).

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check (n.2)

"pattern of squares in alternating colors," c. 1400, short for checker (n.1). As a fabric having such a pattern from 1610s.

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ring-tailed (adj.)

"having the tail ringed with alternating colors," 1725 in ornithology, by 1729 in zoology, from ring (n.1) + tail (n.).

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clonus (n.)

"violent muscular spasms, rapidly alternating contraction and relaxation of a muscle," 1817, from Modern Latin, from Greek klonos "turmoil, any violent motion; confusion, tumult, press of battle," a word of uncertain origin. Related: Clonicity.

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pitter-patter (n.)

early 15c., "rapid repetition of words," from a rhyming reduplication of patter (v.2). As "alternating light beating sounds," 1670s, from patter (v.1). As a verb in this sense by 1708. Compare pit-a-pat.

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reciprocal (adj.)

1570s, "existing on both sides, exclusive or interchangeable" (of duties, etc.), with -al (1) + stem of Latin reciprocus "returning the same way, alternating," from pre-Latin *reco-proco-, from *recus (from re- "back;" see re-, + -cus, adjective formation) + *procus (from pro- "forward," see pro-, + -cus). Related: Reciprocally.

From 1590s as "given, felt, or shown in return;" c. 1600 as "corresponding or answering to each other, mutually equivalent." The sense of "moving backward and forward, having an alternating back and forth motion" (c. 1600) is obsolete. The noun meaning "that which is reciprocal" (to another) is from 1560s. In scientific and mechanical uses, reciprocating, reciprocative (1804), and reciprocatory (1826) have been tried.

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