fretless (adj.)youdaoicibaDictYouDict[fretless 词源字典]
1878, "without annoyance," from fret (n.) (see fret (v.)) + -less. By 1962 as "without frets" (of a banjo, guitar, etc.), from fret (n.2).[fretless etymology, fretless origin, 英语词源]