What it means
To belt out a song is to sing it loud and unapologetic, with your whole chest in it. It's less about being pitch-perfect and more about going for it properly. You hear it at karaoke, in pubs, at matches, or anywhere people stop caring about dignity and start singing like the room owes them a chorus.
Usage examples
"At karaoke down the local, Gaz belted out Sweet Caroline, arms round strangers, mic peaking, landlord wincing, but the whole place still sang along."
"By the third song she was belting out the chorus so loud the people two tables over started filming her."
"Soon as Mr Brightside came on, they were all belting it out like the pub roof owed them money."
"She can't really hold a note, but after two ciders she was belting out every chorus like she was headlining Glasto."
"He was belting out Oasis on the walk home at two in the morning like the whole street had bought a ticket."
Where it comes from
This comes from belt in the sense of hit hard or strike with force, a meaning English has had since the 19th century. From there, belt out grew into sing with force and volume. The out adds that full-on, no-holding-back feeling, like you're really letting the song loose.
