Origin and history of lock-step

"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). Phrase under lock and key attested from early 14c. In firearms, the part of the mechanism which holds 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" (1806).

As a Man that sees a screw'd Gun shot off, though he may not be able to describe the number, bignesse, shape and coaptation of all the Pieces of the Lock, Stock and Barrel, yet he may readily conceive that the Effects of the Gun, how wonderful soever they may seem, may be perform'd by certain pieces of Steel or Iron, and some parcels of Wood, of Gun powder, and of Lead, all fashion'd and put together according to the exigency of the Engine, and will not doubt, but they are produc'd [...] without the assistance of spiritual or supernatural Agents. [Robert Boyle, Some Considerations Touching the Usefulnesse of Experimental Natural Philosophy, 1664.]

"a completed movement of the foot in walking," Old English steppa (Mercian), stæpe, stepe (West Saxon) "an act of stepping; a footprint," also "a stair, rung of a ladder," and by late Old English "manner of walking," from the source of step (v.). Compare Old Frisian, Middle Dutch, Dutch stap, Old High German stapfo, German Stapfe "footstep").

Also "distance traversed by a step" (mid-13c.), which led to the use of step as a unit of linear measure (late 13c.) equivalent to about 2.5 feet or a half-pace. In dancing, from 1670s. The specific meaning "type of military pace" is from 1798.

It is attested from late Old English as "degree on a scale, degree in progress or advance," in virtue, sin, etc. The notion is perhaps of a step as a support for the foot in ascending or descending. The figurative meaning "action which leads toward a result, a proceeding in a series of them" is recorded from 1540s.

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 "follow the example of" is from mid-13c.