A person who is said to have a stiff upper lip displays fortitude and stoicism in the face of adversity, or exercises great self-restraint in the expression of emotion.
[1][2] The phrase is most commonly heard as part of the idiom "keep a stiff upper lip", and has traditionally been used to describe an attribute of British people in remaining resolute and unemotional when faced with adversity.
The idea of the stiff upper lip is traced back to Ancient Greece – to the Spartans, whose cult of discipline and self-sacrifice was a source of inspiration to the English public school system; and to the Stoics.
[1] Poems that feature a memorable evocation of Victorian stoicism and a stiff upper lip include Rudyard Kipling's "If—" and W. E. Henley's "Invictus".
Such schools were heavily influenced by stoicism, and aimed to instil a code of discipline and devotion to duty in their pupils through 'character-building' competitive sports (as immortalised in the poem "Vitai Lampada"), corporal punishments and cold showers.