Mārs

Mars was the Roman god of war, second only to Jupiter in importance to the Roman state — a fitting priority for a civilization built on military conquest. His name's etymology is uncertain and hotly debated: the most traditional derivation connects Mars to an older Latin deity Mavors or Mamers, possibly from an Oscan or Italic root, though some scholars propose connections to Proto-Indo-European roots meaning 'to shine' or 'to strike.' What is certain is that Mars was not simply a god of violence: he was the patron deity of the Roman army, of the Roman people themselves (who were called the children of Mars through his mythological fathering of Romulus and Remus), and of the agricultural season — the month of March, named for him, was both the beginning of the military campaigning season and the beginning of the agricultural year.

The adjective martialis — of or belonging to Mars — was used in Latin to describe anything connected to the god of war: martial music, martial exercises, martial law. Marcus Valerius Martialis, the 1st-century CE epigrammatist known to English readers as Martial, bore a cognomen (additional name) derived from the same root. In medieval astrological thought, Mars governed the choleric temperament: hot, dry, irascible, aggressive, quick to anger. Those born under Mars's influence were martial in disposition — warriors, surgeons, and smiths, all workers in blood and iron.

Martial entered English in the 14th century and has never lost its core meaning. Martial arts, martial law, martial music, court martial — the adjective has maintained its military specificity more faithfully than its astrological siblings jovial and mercurial, which have drifted toward more general descriptive use. The reason may be that warfare has remained a consistent presence in human affairs, providing constant reinforcement for the word's precise meaning. You know what martial means because the thing it describes has never stopped happening.

Mars also gave his name to Tuesday — through a chain of translation that reveals how systematically Germanic cultures mapped their own deities onto Roman ones. Latin dies Martis (day of Mars) became French mardi and Spanish martes. The Germanic tribes associated Mars with their own war god Tiw (or Tyr in Norse), and so Tuesday — Tiwesdæg in Old English — is the day of Tiw, who was Mars's functional equivalent. The Roman god of war controls not only the English adjective for military affairs but also one-seventh of every English speaker's week.