different between inappreciative vs thankless

English

in- +? appreciative

inappreciative (comparative more inappreciative, superlative most inappreciative)

unappreciative.
- 1860, George Eliot, Mill on the Floss, Book II, Chapter IV, [1]
  […] he found Mr. Poulter, with a fixed and earnest eye, wasting the perfections of his sword-exercise on probably observant but inappreciative rats.
- 1949, Sinclair Lewis, The God-Seeker, New York: Popular Library, Chapter 31, pp. 171-2,
  She was the support and the terror of the pastor, punctual at meeting and critical of the girls in the choir and the pastor's wife's clothes, mighty in making flannels for the poor but inappreciative of their adulteries […]

From Middle English thankles, equivalent to thank +? -less.

thankless (not comparable)

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