"Flicker," meaning to faint, comes from the flickering of a light, "battering" (begging) from knocking at back doors, and "bull" (policeman) from the plunging, bullying attitude of these officers when dealing with rowdies.
A number of words used by tramps are also in vogue among criminals, who are even more in need of a secret language than are vagabonds. It must be remembered, however, that in America at least, and to some extent in other countries as well, a great many tramps are merely discouraged criminals, and it is not unnatural that they should cling to expressions which they found valuable when they were more intimately connected with criminal life. Even as tramps they are continually making the acquaintance of criminals, and it is one of their main delights to be seen in the company of notorious thieves and burglars; they enjoy such companionship as much as certain "middle-class" people enjoy the society of "aristocrats."
The word "elbow," meaning detective, is one of the slang terms common among both hoboes and criminals. It comes from the detective's habit of elbowing his way through a crowd, and it is the gloomiest word, as I heard a hobo once say, that the outcast ever hears by way of warning. Be he beggar or thief, a shiver invariably runs down his spinal column if a pal whispers or shouts "Elbow" to him while he is "at work" in a public thoroughfare. The word "finger," which is synonymous with "bull," has very nearly, but not quite, the same effect, because the finger is in uniform, whereas the elbow prowls about in citizen's clothes. "Finger" comes from the policeman's supposed love of grabbing offenders. "They like to finger us," a hobo said to me, one night, in a Western town where we were both doing our best to dodge the local police force. "Some people calls 'em the eye o' the law, but that ain't what they is; they're the finger o' the law."
"Revolver," or "repeater," is both a tramp and a criminal term for the professional offender, the man who is continually being brought up for trial. "Lighthouse" is one of the most picturesque words in the lingo. It means a man who knows every detective of a town by sight, and can "tip them off" to visiting hoboes and criminals. As mariners at sea look for the beacon light which is to guide them safely into harbor, so tramps and criminals look for their "lighthouse." He is one of the most valuable acquaintances in the outcast world, and more advice is taken from him than from any other inhabitant. Such expressions as a "yellow one" for a gold watch, a "white one" for a silver watch, a "leather" for a pocket-book, and a "spark" for a diamond, explain themselves, and I have heard them used by others than those in criminal life; but they are distinctly lingo terms.