To sing very loudly and powerfully, or to hit someone or something hard, or the straps or bands worn around the waist.
From 'belt' (from Old English 'beltan'—to gird or bind), which also relates to the leather strap and the loud singing style of operatic performers.
Belting in singing is literally about forcing air from your diaphragm with power—it's the complete opposite of crooning. Musical theater made 'belting' famous as a super-impressive vocal technique.
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