'self-possession' is a compound of 'self' and 'possession'. 'self' originates from Old English 'self', from Proto-Germanic '*selbaz', meaning 'one's own person'. 'possession' originates from Latin 'possessio', from the verb 'possidēre' meaning 'to have, to hold'.

'self' remained in English from Old English into Modern English unchanged in basic sense; 'possession' passed into English via Old French and Middle English from Latin 'possessio'. The compound 'self-possession' arose in English by combining these elements, and by the 17th–18th century was used to mean calmness or composure.

Originally the elements could suggest literal 'ownership or control of oneself'; over time the compound came to be used primarily in the figurative sense 'composure, calm self-control'.