sepoy

English sepoy comes from Portuguese sipai, borrowed from Urdu/Hindi sipāhī, itself from Classical Persian sipāhī ("soldier, horseman"), a derivative of sipāh ("army").

The word began as a general Persian term for a soldier, tied to the word for an army itself, before Portuguese traders picked it up in South Asia and passed it into English, where it settled specifically as the name for Indian soldiers serving under European command.

It's a doublet of spahi and sipahi — words that trace back to the same Persian root but arrived in English by different routes.