In Athens around 300 BC, Zeno of Citium founded a school of philosophy called Stoicism, and a person who followed the idea was called a Stoic. The main goal of a Stoic was to avoid unnecessary emotions.
Stoics believed that certain feelings, like joy and sorrow, were destructive to people and should be avoided and that a virtuous life could result in peace and harmony with nature. The adjective Stoic describes something that relates to this particular school of philosophy. Major Stoic philosophers included Marcus Aurelius and Seneca the Younger. Stoic is always capitalized.