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Douglas Harper's Etymology Dictionary
cliff-hanger

also cliffhanger, "suspenseful situation," 1937, in reference to U.S. cinema serials, agent noun from cliff + agent noun from hang (v.). In some cases, especially Westerns, the hero or heroine literally was dangling from a cliff at the end of an episode.

Wiktionary
cliff-hanger

n. (alternative spelling of cliffhanger English)