To sing a song with
gusto and a certain degree of
competence, though perhaps not with the style or
finesse of a
top-notch singer. Might be used to describe the singing of a performer known for being skilled at some
other discipline, such as
dance or
comedy.
Presumably the phrase is related to using one's
abdominal muscles to give power to the singing voice.
ex.: "Starring in the
The Music Man,
Tim Robbins showed that he could really
belt out a tune." (
fictitious)
var.:"belt one out"