lock-step n.

"means of fastening," Old English loc "bolt, appliance for fastening a door, lid, etc.; barrier, enclosure; bargain, agreement, settlement, conclusion," from Proto-Germanic *lukana-, a verbal root meaning "to close" (source also of Old Frisian lok "enclosure, prison, concealed place," Old Norse lok "fastening, lock," Gothic usluks "opening," Old High German loh "dungeon," German Loch "opening, hole," Dutch luik "shutter, trapdoor").

Ordinary mechanical locks work by means of an internal bolt or bar which slides and catches in an opening made to receive it. "The great diversity of meaning in the Teut. words seems to indicate two or more independent but formally identical substantival formations from the root" [OED]. The Old English sense "barrier, enclosure" led to the specific meaning "barrier on a stream or canal" (c. 1300), and the more specific sense "gate and sluice system on a water channel used as a means of raising and lowering boats" (1570s).

From 1540s as "a fastening together," hence "a grappling in wrestling" (c. 1600). In firearms, the part of the mechanism which explodes the charge (1540s, probably so called for its resemblance to a door-latching device), hence figurative phrase lock, stock, and barrel (which add up to the whole firearm) "the whole of something" (1842). Phrase under lock and key attested from early 14c.

Old English steppa (Mercian), stæpe, stepe (West Saxon) "stair, act of stepping," from the source of step (v.). Compare Old Frisian, Middle Dutch, Dutch stap, Old High German stapfo, German Stapfe "footstep").

It is attested from late Old English as "degree on a scale." The figurative meaning "action which leads toward a result" is recorded from 1540s. In dancing, from 1670s. The meaning "type of military pace" is from 1798.

The warning phrase watch your step is attested from 1911 (Wycliffe, late 14c., has keep thy foot in essentially the same sense). Step by step indicating steady progression is from 1580s. To follow in (someone's) steps is from mid-13c.