before vowels therm-, word-forming element meaning "hot, heat, temperature," used in scientific and technical words, from Greek thermos "hot, warm," therme "heat" (from PIE root *gwher- "to heat, warm").
late 13c., "two of the same kind or class connected or considered together," especially "a man and a woman associated together by marriage or love," from Old French cople "married couple, lovers" (12c., Modern French couple), from Latin copula "tie, connection," from PIE *ko-ap-, from *ko(m)- "together" + *ap- "to take, reach."
From mid-14c. as "that which unites two." In electricity, "pair of connected plates of different metals used for creating a current," from 1863.
<a href="https://www.etymonline.com/word/thermocouple">Etymology of thermocouple by etymonline</a>
Harper, D. (n.d.). Etymology of thermocouple. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/thermocouple