Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."
1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat, feverish heat," from PIE root *gwher- "to heat, warm." Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.
<a href="https://www.etymonline.com/word/athermanous">Etymology of athermanous by etymonline</a>
Harper, D. (n.d.). Etymology of athermanous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/athermanous