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
Presumably the phrase is related to using one's abdominal muscles
to give power to the singing voice.
"Starring in the The Music Man
, Tim Robbins
showed that he could really belt out a tune
"belt one out"