Wiktionary

1. a hundred and ten percent(Noun)

   A level of effort exceeding one's sustained capacity, possibly risking injury.

2. a hundred and ten percent(Noun)

   The exertion of more than seems possible, hence 110%, not 100%, the usual maximum amount possible.

Origin: Based on the metaphor of operating a machine at higher than its rated capacity or exceeding the normal maximum of some rated parameter such as speed or temperature.