c. 1600, "part of the Italian mainland ruled by Venice," from Modern Latin terra firma, literally "firm land," from Latin terra "earth, land" (from PIE root *ters- "to dry") + firma, fem. of firmus "strong, steadfast" (from suffixed form of PIE root *dher- "to hold firmly, support"). Meaning "the land" (as distinct from "the sea") is first attested 1690s. Hakluyt and Sandys also used English firm (n.) to mean "the firm land, the mainland, terra firma."
