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.
of persons, "weakened and disordered or agitated by shocks," 1640s, past-participle adjective from shake (v.). By 1725 as "put into quick or vigorous alternating motion."
"pattern of squares in alternating colors," c. 1400, short for checker (n.1). As a fabric having such a pattern from 1610s.
"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.