'extragalactic' originates from the Latin prefix 'extra' meaning 'outside' or 'beyond' and the element 'galactic' ultimately from Greek 'galaxias' (from 'gala' meaning 'milk'), referring to a galaxy.

'galaxy' comes from Greek 'galaxias' (related to 'gala', 'milk'), which entered Late Latin and then Middle English; 'galactic' was formed from these roots and the adjective 'extragalactic' was coined in modern scientific English (20th century) by adding Latin 'extra-' to 'galactic'.

Initially the roots referred literally to the 'Milky Way' or 'milky circle'; in modern scientific usage 'extragalactic' specifically denotes being outside the Milky Way galaxy.