His ear was severed by a sword wielded by his friend, the painter, Paul Gauguin, in a drunken row over a woman called Rachel and the true nature of art.
Gauguin lied about the incident and fled, two German art historians now believe. Van Gogh covered up to protect his friend and was placed in a mental institution.

[…]

Nina Zimmer, curator of a large Van Gogh exhibition at the Kunstmuseum
in Basle until September, is unconvinced. “Maybe they are right,” she
said. “But almost any theory is plausible because there are so few
established facts.” — John Lichfield