Metejón is a fascinating word that describes a strong, often fleeting, infatuation or crush. It's more intense than a simple liking but less profound than deep love, capturing the passionate yet sometimes irrational nature of a sudden romantic obsession.
In "Por Una Cabeza," the word is used to describe the singer's intense, almost addictive, attraction to a "coqueta y risueña mujer" (flirtatious and smiling woman). It perfectly encapsulates the song's theme of being swept away by a powerful, almost gambling-like, desire, much like the thrill and risk of a horse race.
This is the old Carlos Gardel tango standard, and IL DIVO sing it here as a full orchestral showpiece rather than a strict ballroom track. The whole song runs on a horse-racing image: "Por una cabeza" (By a head) is the exact margin a horse loses a race by, and the singer uses it as his measuring stick for every bad decision he's made for love. He's a gambler admitting he knows better and does it anyway.
The trick of the lyric is that it never separates the racetrack from the woman. He calls one lover "De un noble potrillo" (Of a noble foal), and later admits "Si ella me olvida" (if she forgets me), he'd lose everything a thousand times over and asks, straight out, "¿Para qué vivir?" (why go on living?). That's not subtle, and it isn't meant to be. He knows the bet is rigged against him and he's placing it again anyway, which is the whole joke and the whole ache of a good tango.