Definition of "fretless"

You would live longer and happier if you would only be quiet and fretless.

1895, Virginia Reed, Daily Cheer for All the Year, Philadelphia: G.W. Jacobs & Co., page 18

Will the fretless world be happy or will restless nature shout / For some old-time fret or worry just to rave and kick about?

1907 August, J. W. Foley, quotee, “The Menace of Reform”, in Texas Medical News, volume 16, number 10, Austin: Texas Medical News Publishing Co., page 525

Is it not worth while to try and approximate, if we cannot hope to attain, the ideal of fretless, fussless, and unworrying poise?

1913, George Lincoln Walton, Calm Yourself, Boston: Houghton Mifflin, page 7

Read's creations have covered just about all of bass-dom, from vintage-style 4-strings to 32-position fretlesses to 25-fret 8-strings tuned F♯BEADGCF.

2003, Jim Roberts, American Basses, San Francisco: Backbeat Books, page 150

No other machine can produce sounds which are so often described as 'singing', 'purring', 'humming', and 'ringing'. But this array of mellifluous notes doesn't come cheaply: even those who have mastered the fretless to moderate levels of proficiency have been obliged to spend many hundreds of hours on techniques which fretted-bass players either take for granted or simply do not require.

2006, Adrian Ashton, The Bass Handbook, San Francisco: Backbeat Books, page 47

Many fretlesses, especially less expensive models that mimic a fretted counterpart, have fretted-bass nut slots.

2008, Jonathan Hererra, “ASK BP”, in Chris Jisi, editor, Bass Player Presents The Fretless Bass‎, Milwaukee: Backbeat Books