On the Origin of “Sentient”
The first published use of the word "sentient," according to the Oxford English Dictionary, was in 1603 by Philemon Holland in Plutarch’s Philosophie, commonlie called, the Morals (you can buy the first complete edition in English for USD 11,000 or EUR 7,414 here).
Intelligence is the motion of the intelligence about that which is stable..: but opinion is the mansion of the sentient about that which moveth.
That doesn’t help much, but in 1632 (in the second edition of John Guillim’s A Display of Heraldrie, if you must know), we get a bit more help.
Forasmuch as God would that the faculties both intelligent and sentient should predominate in the head [etc.].
"Sentient" was considered a faculty that God put in our head (or brain, depending on which reference you look at). It is other-than intelligence, and shall prove, through about 1879, to be an adjective consistently have something to do with the capacity to feel.
1. That feels or is capable of feeling; having the power or function of sensation or of perception by the senses.
Beginning in 1815, however, a change in direction displayed itself, with Robert Southey, Elizabeth Barrett Browning and others using it to mean:
Conscious or percipient of something.
(Mrs. Browning wrote: The poet’s sight grew sentient Of a strange company around in 1844, for instance.)
In 1839, "sentience" came on the scene in Edgar Allan Poe’s Fall of the House of Usher:
This opinion [of Usher's], in its general form, was that of the sentience of all vegetable things.
Yes, vegetables. "Sentience," too, at the beginning, was used strictly to mean the capacity to sense.
"Sentient" briefly progressed from conscious (as in awareness), to a hint of a larger, less elementary conception of consciousness. In 1886, Frederic W. H. Myers wrote:
The insentient has awoke..into sentiency; the sentient into the fuller consciousness of human minds.
Since then, "sentient" has been moving closer to "consciously perceiving," to which now we would add (at least) "pleasure and pain."
"Sentience" has toyed with evolving into something more complex involving consciousness, and only time will tell what will occur. Consciousness is probably the endgame of most discussions of states of being, and we are likely to never find the answer, or agree to, what it is, where it is, who has it and to what degree, and whether or not it should matter in discussions of sentience, suffering and rights.