Who were "sepoys" and what was the last straw that led them to rebel in 1857?

Sepoys were musket-wielding Indian infantrymen who were not happy with the East Indian Company's rules and imposed culture. The British employed the sepoys in their armies and by 1857 there were around 300,000 sepoy soldiers in contrast to 50,000 British soldiers. The final straw was when the new muskets the soldiers were supposed to use incorporated beef tallow to help the bullets leave the chamber. For many of the Indian soldiers cows were considered sacred and this provided the tipping point for rebellion. The rebellion was widespread and many people were killed, especially Indian civilians. In the end, the British retaliation was swift and brutal, and most of the sepoy units were disbanded. Interestingly, sepoy became a derogatory term for the Irish who also resisted British rule.

https://en.wikipedia.org/wiki/Indian_Rebellion_of_1857