Five thousand years ago, the Proto-Indo-Europeans had a root for standing: *steh₂-. It may be the most productive root in the English language. From it came stand, state, station, stable, static, status, statue, establish, estate, and steady.
Steady entered English as stede plus the suffix -y — literally 'having a firm place'. Old English stede meant 'place' or 'position', and its descendants are everywhere: homestead (a home-place), farmstead (a farm-place), instead (in the place of), and steadfast (fixed in place). A steady person is one who holds their position.
The word's history mirrors a human preoccupation with firmness. To be unsteady is to be unreliable — physically, morally, emotionally. 'Going steady' in the romantic sense (first attested in 1905) meant committing to one person, holding position rather than wandering.
German stetig ('constant, continuous') is the closest cognate, preserving the sense of regularity. Scandinavian forms — Swedish stadig, Danish stadig — carry the same weight of immovable calm.
The command 'Steady!' has served on battlefields and operating theatres alike. In both contexts, the instruction is the same: hold your place, do not waver, trust the ground beneath you.