1
general::
1. Knock unconscious; beat up, trounce; murder. For example, The police officer was accused of belting out the teenager before taking him to the station, or The hold-up man belted out the storekeeper and fled with the money. This expression originated in boxing. [Slang; c. 1940] 2. Sing or play music very loudly, as in She belted out the national anthem before every game. [Colloquial; c. 1950]
American Heritage Idioms
2
general::
Phrase(s): belt something out
Fig. to sing or play a song loudly and with spirit. • When she’s playing the piano, she really belts the music out. • She really knows how to belt out a song.
McGrawhill's American Idioms And Phrasal Verbs