The English word "companion" hides a small, vivid picture inside it. Take it apart and you find com- ("with") and panis ("bread"). A companion is, in the most literal sense, "one with whom you share bread."
A Vulgar Latin Coinage
The compound itself, companiō, does not appear in classical Latin. It seems to have been coined in Vulgar Latin — the everyday speech of soldiers, traders, and common people across the late Roman Empire — as a translation of a Germanic idea. Gothic gahlaiba meant exactly the same thing: ga- ("with") plus hlaifs ("loaf"). When Roman armies and Germanic tribes ran into each other, the loaf-sharers became companions.
From Mess Hall to Best Friend
By the medieval period, compagnon in Old French simply meant a friend, a comrade-in-arms, or a fellow traveller. English borrowed it in the 14th century. By Shakespeare's time, "companion" could be a tender word for a close friend or a slightly contemptuous one for a hanger-on — the same range it carries today.
A Family Built on Bread
The Latin panis is the root of a small loaves-and-fishes family in English: pantry (originally a place for bread), panini, pannier (a basket for bread), and the somewhat formal pan of frying pan — though that last one came from a different Latin route. The word "company" itself is a sibling of "companion," from the same root: a company is a group of people who have, in principle, eaten together.