1520s, "firmly fixed in place or station" (replacing earlier steadfast), from stead + adjectival suffix -y (2), perhaps on model of Middle Dutch, Middle Low German stadig. Old English had stæððig "grave, serious," and stedig "barren," but neither seems to be the direct source of the modern word. Old Norse cognate stoðugr "steady, stable" was closer in sense. As an adverb from c. 1600.
Originally of things; of persons or minds from c. 1600. Meaning "working at an even rate" is first recorded in 1540s. Steady progress is etymologically a contradiction in terms. Steady state first attested 1885; as a cosmological theory (propounded by Bondi, Gold, and Hoyle), it is attested from 1948. Related: Steadily.