It is the hypothetical source of/evidence for its existence is provided by: Sanskrit tarsayati "dries up;" Avestan tarshu- "dry, solid;" Greek teresesthai "to become or be dry," tersainein "to make dry;" Latin torrere "dry up, parch," terra "earth, land;" Gothic þaursus "dry, barren," Old High German thurri, German dürr, Old English þyrre "dry;" Old English þurstig "thirsty."
"of the nature of or abounding in water," 1640s, from Latin aqua "water" (from PIE root *akwa- "water") on analogy of French aqueux "watery" (16c., which, however, is from Late Latin aquosus "abounding in water"). Or perhaps it is by analogy of Latin terreus "earthy," from terra "earth." Aqueous humor "watery fluid in the surface of the eye" is the original use in English. Related: Aqueousness.
<a href="https://www.etymonline.com/word/terraqueous">Etymology of terraqueous by etymonline</a>
Harper, D. (n.d.). Etymology of terraqueous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/terraqueous