martyr

Martyr comes from Middle English martir, from Old English martyr, borrowed from Ecclesiastical Latin martyr, which came from Ancient Greek mártur, a later form of mártus, meaning 'witness.'

The Greek root simply meant a person who testifies to something they've seen. As it moved into Ecclesiastical Latin and then English, the word narrowed to describe those who testified to their faith through death.

The facts don't point to a surviving English cognate beyond the word's own path through Old English and Latin.