also bell-wether, "lead sheep (on whose neck a bell was hung) of a domesticated flock," mid-14c. (late 13c. in Anglo-Latin; late 12c. as a surname), from bell (n.) + wether. Figurative sense of "chief, leader" is from mid-14c.
"hollow metallic instrument which rings when struck," Old English belle, which has cognates in Middle Dutch belle, Middle Low German belle but is not found elsewhere in Germanic (except as a borrowing); apparently from PIE root *bhel- (4) "to sound, roar" (compare Old English bellan "to roar," and see bellow).
As a division of daily time aboard a ship, by 1804, from its being marked by bells struck every half hour. Statistical bell curve is by 1920, said to have been coined was coined 1870s in French. Of glasses in the shape of a bell from 1640s. Bell pepper is from 1707, so called for its shape. Bell, book, and candle is a reference to a form of excommunication (the bells were rung out of order and all together to signify the loss of grace and order in the soul of the excommunicated).
To ring a bell "awaken a memory" (1934) is perhaps a reference to Pavlovian experiments; it also was a signal to summon a servant (1782).
"male sheep," especially a castrated one, Old English weðer "ram," from Proto-Germanic *wethruz (source also of Old Saxon wethar, Old Norse veðr, Old High German widar, German Widder, Gothic wiþrus "lamb"), literally "yearling," from PIE root *wet- (2) "year" (source also of Sanskrit vatsah "calf," Greek etalon "yearling," Latin vitulus "calf," literally "yearling").
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Definitions of bellwether from WordNet
someone who assumes leadership of a movement or activity;