1792, Robert Sadler, Wanley Penson, or The Melancholy Man, London: C. & G. Kearsley, Volume 3, p. 252,
There are moments, indeed, in which I could be pleased to repay even a zestless joke with a smile, and, to feed the cheerfulness of a companion, rummage my own recollection for a mirthful incident; but, alas! ’tis not so now—My soul is too much absorbed in its own gloomy ruminations, to be drawn forth by its accustomed urbanity […]
Source: wiktionary
1865, Charles Heavysege, Jephthah’s Daughter, Montreal: Dawson Brothers, IV, (unpaginated),
[…] So he passes
To second childhood; but, as quickening gases,
Being fled, leave zestless a once cheering draught,
We grow not merry though the Dotard laughed.
Source: wiktionary
As he descended the stairs he felt calmer, readier to do battle with the forces arrayed against him; and that made him feel a little warm towards the weak healthy boy who never did battle at all, but just drank and debauched himself in a bored, zestless way.
Source: wiktionary
c. 1956, Martin Luther King Jr., sermon delivered at the time of the Montgomery bus boycott, cited in Dream: The Words and Inspiration of Martin Luther King, Jr., Boulder, CO: Blue Mountain Press, p. 81,
Courageous men never lose the zest for living even though their life situation is zestless; cowardly men, overwhelmed by the uncertainties of life, lose the will to live.
Source: wiktionary