Origin and history of -mancy

late 14c., "mental derangement characterized by excitement and delusion," from Late Latin mania "insanity, madness," from Greek mania "madness, frenzy; enthusiasm, inspired frenzy; mad passion, fury," related to mainesthai "to rage, go mad," mantis "seer," menos "passion, spirit," all of uncertain origin, perhaps from PIE *mnyo-, suffixed form of root *men- (1) "to think," with derivatives referring to qualities and states of mind or thought.

Mania is manifested by psychic elevation, increased motor activity, rapid speech and the quick flight of ideas. [Scientific American, September 1973]

Sense of "fad, craze, enthusiasm resembling mania, eager or uncontrollable desire" is by 1680s, from French manie in this sense. Sometimes nativized in Middle English as manye. Used since 1500s as the second element in compounds expressing particular types of madness (such as nymphomania, 1775; kleptomania, 1830; megalomania, 1890), originally in Medical Latin, in imitation of Greek, which had a few such compounds, mostly post-classical: gynaikomania (women), hippomania (horses), etc.

"divination by means of a cock and grains of corn," 1680s, from -mancy "divination" + Latinized form of Greek alektryon "cock," literally "warder-off, fighter," related to alexein "to ward off, drive or keep off" (see Alexander, and compare Alekto, name of one of the three Furies). Perhaps originally a personal name, applied at first to the fighting cock, then to cocks generally. The earlier form of the word in English was alectoromancy (1650s). Letters of the alphabet were traced on the ground and a grain of corn was placed on each.