To let out one’s frustration or anger, usually by shouting. The term comes from the early days of railroading, when locomotives had no safety valves. When the steam pressure built up, the engineer would pull a lever that would blow off steam and prevent an explosion. It was transferred to human wrath in the early nineteenth century. “The widow . . . sat . . . fuming and blowing off her steam,” wrote Frederick Marryat (The Dog-Fiend, 1837). See also let off steam.