Thumbs down was invented by a painter in 1872.
Roman sources only say "pollice verso", with a turned thumb – which gesture meant which outcome is unclear. Our mental image comes from Jean-Léon Gérôme's 1872 painting showing Vestals with lowered thumbs. Nor did every loser die: Emperor Augustus banned fights without the option of mercy.
Useful: With ancient gestures, check whether the source is ancient or from an art gallery.
Source: en.wikipedia.org