1875, "place a flag on or over," from flag (n.1). Meaning "designate as someone who will not be served more liquor," by 1980s, probably from use of flags to signal trains, etc., to halt, which led to a verb meaning "inform by means of signal flags" (1856, American English). Meaning "to mark so as to be easily found" is from 1934 (originally by means of paper tabs on files). Related: Flagged; flagger; flagging.
1890, of a railroad ticket, "mark for use on a certain route," from route (n.). The meaning "direct (an electrical signal, phone call, etc.) over a particular circuit or to a particular location" is by 1948. Related: Routed; routing; routeing (1881).
"one who or that which responds or replies," by 1834 of persons; 1867 of devices (telegraphy); agent noun from respond (v.). Meaning "device which automatically retransmits a pulse or signal" is by 1945.
early 14c., curfeu, "evening signal, ringing of a bell at a fixed hour" as a signal to extinguish fires and lights, from Anglo-French coeverfu (late 13c.), from Old French cuevrefeu, literally "cover fire" (Modern French couvre-feu), from cuevre, imperative of covrir "to cover" (see cover (v.)) + feu "fire" (see focus (n.)). Related: Curfew-bell (early 14c.).
The medieval practice of ringing a bell (usually at 8 or 9 p.m.) as an order to bank the hearths and prepare for sleep was to prevent conflagrations from untended fires. The modern extended sense of "periodic restriction of movement" had evolved by 1800s.