More from the BBC adaptation of Charles Dickens' Bleak House. In this particular scene, one character, Sergeant George, is infuriated at another character, Mr. Smallwood, his petty landlord come to turn him out of his place of business; all the while Smallwood is reneging on a past agreement between the two men. The situation quickly escalates and George, a military man, and a man of honor, threatens Mr. Smallwood by leveling a pistol at him, causing Smallwood to cry out "steady on!", and hurriedly retreat from his mission of eviction.
So my question is: What does this phrase, "steady on", mean? I got from context that it would cause someone angry to stop what they're doing, but does it mean something more exact outside this particular context? If it does in fact mean something, how do you get from the ambiguous "steady on" to the actual meaning of the phrase — what's the thinking behind the "steady", for example? And finally, would the phrase be current today?