Yours, Etc.: Origins and Uses of 8 Sign-Offs
1530s, from Latin tropus "a figure of speech," from Greek tropos "turn, direction, turn or figure of speech," related to trope "a turning" and trepein "to turn," from PIE root trep- "to turn" (cf. Sanskrit trapate "is ashamed, confused," properly "turns away in shame;" Latin trepit "he turns"). Technically, in rhetoric, a figure of speech which consists in the use of a word or phrase in a sense other than that which is proper to it.
in medieval church music, melody, explicatory text, or both added to a plainchant melody. Tropes are of two general types: those adding a new text to a melisma (section of music having one syllable extended over many notes); and those inserting new music, usually with words, between existing sections of melody and text.