'pro-extraterrestrial' originates from Latin, specifically the prefix 'pro' (Latin 'pro') where 'pro' meant 'for', and from 'extraterrestrial' which ultimately comes from Latin elements 'extra' and 'terrestris' ('terra') meaning 'outside' and 'earth' respectively.

'extraterrestrial' changed from Late Latin 'extraterrestrialis' (built from 'extra' + 'terrestris') and eventually became the modern English word 'extraterrestrial'; the prefix 'pro' comes directly from Latin 'pro' and the compound 'pro-extraterrestrial' is a modern English formation combining these elements.

Initially the components meant 'for' and 'outside the Earth'; over time the combined modern expression has been used to mean 'in favor of extraterrestrial beings or contact' rather than a literal locational description.