Definitions

1. interjection

an expression of impatience or annoyance, especially when someone is being slow or awkward, often used by children

“She exclaimed, ‘Stoppit!’ as her brother continued to trip over his own feet.”

2. verb

to stop or interrupt something, especially in a sudden or impatient manner

“He shouted ‘Stoppit!’ to his friend, who was about to walk into a puddle.”