Metejón is a fascinating word that translates to "infatuation" or a "crush." It's a colloquial term, particularly common in the Río de la Plata region (Argentina and Uruguay), where tango originated.
In "Por Una Cabeza," Carlos Gardel uses metejón to describe a fleeting, intense, and often irrational passion, comparing it to the unpredictable nature of horse racing. He sings about a "metejón de un día" (a one-day infatuation), perfectly capturing the song's theme of passionate, yet ultimately risky and transient, loves and gambles.
Carlos Gardel recorded this in 1935 with the lyricist Alfredo Le Pera, for the film Tango Bar, and it turns a day at the racetrack into an excuse to talk about a woman he can't get over. The title comes straight from the track: "Por una cabeza, de un noble potrillo" ["By a head, of a noble colt"] is the exact margin by which a favorite loses a race, and from there every line about horses is really about her. He tells himself "No olvides, hermano" ["Don't forget, brother"], "Vos sabes, no hay que jugar" ["You know, don't gamble"], and breaks the promise the moment she crosses his path again: "Pero si un mirar me hiere al pasar" ["But if a glance wounds me as it passes"], "Su boca de fuego" ["Her mouth of fire"], "Otra vez quiero besar" ["I want to kiss again"].
By the last verse he's dropped the racing talk and asks outright: "Si ella me olvida" ["If she forgets me"], "Qué importa perderme" ["what does it matter if I lose"] "Mil veces la vida" ["my life a thousand times"], "Para qué vivir" ["why live"]. That's a strange thing to sing over a beat you can waltz to, which is probably why the melody keeps showing up behind the tango scene in whatever film needs one — Scent of a Woman and True Lies both borrowed it. Gardel plays a man who already knows he'll bet on her again and lose again, and sounds almost proud of it.