ters-
“to be dry, thirst”Source of Latin terra, English thirst, terrain, toast
Root for dryness, yielding Latin terra (dry land/earth) and English thirst, terrain, torrid.
Discussion
The PIE root *ters- (to be dry, to be thirsty) produced the vocabulary of dryness, thirst, and land — connecting the physical sensation of a parched throat to the concept of solid ground (land as the dry thing, opposed to sea).
Latin terra (earth, land, ground — the dry substance) continues the root with the sense "the dry thing" and gave English an enormous vocabulary: terrain, territory (the land belonging to a city), terrace (a flat area of dry ground), terrestrial (pertaining to the earth), Mediterranean (medius + terra, "middle of the land"), inter (in-terrāre, to put into the earth — to bury), and subterranean (under the earth). The word terracotta (Italian, "baked earth") names pottery by its material.
The connection between dryness and land is transparent: in a world defined by the opposition of land and sea, the fundamental quality of land is that it is dry. Terra IS the dry place.
Latin torrēre (to dry, to parch, to roast) preserves the active verbal sense: toast (from tostus, "dried/roasted"), torrid (parched with heat), and torrent (torrēns, "boiling/rushing" — water so violent it seems to burn) show the heat-dryness extension.
English thirst (OE þurst, from PGmc *þurstuz) continues the root natively through Germanic — thirst is the body's experience of internal dryness. German Durst and Dutch dorst confirm the cognate set.
Greek térsesthai (τέρσεσθαι, "to become dry") preserves the Hellenic reflex.
Sanskrit tṛ́ṣyati ("he thirsts") and tṛṣṇā́ ("thirst" — the Buddhist concept of taṇhā/tṛṣṇā, "craving," is etymologically "thirst") continue the root in Indo-Iranian. The Buddhist First Noble Truth — that suffering arises from craving (tṛṣṇā) — deploys this PIE root at the foundation of an entire religious philosophy: craving IS thirst, and liberation is the quenching of that thirst.