The Spanish word 'apóstol' (meaning 'apostle') comes from Latin 'apostolus', which was borrowed from Ancient Greek 'apostolos' meaning 'one who is sent'. The Greek word was formed by combining two elements: the prefix 'apo-' meaning 'away from' and the verb 'stellein' meaning 'to send'. So literally, an 'apostolos' was someone who was 'sent away' or 'sent forth' (as on a mission).

This etymology reflects the historical usage of the word to refer to the twelve apostles of Jesus Christ, who were literally 'sent forth' to spread his teachings. The word maintained this meaning as it passed from Greek through Latin and into Spanish.

A related Spanish word is 'apostólico' (apostolic), which directly refers to something relating to an apostle or the apostles. Another related term is 'apostolado', referring to apostleship or apostolic activity.

The English word 'apostle' is a direct cognate of Spanish 'apóstol', as both descended from the same Greek source through Latin. The English words 'epistle' and 'postal' are also related, as they come from the same Greek root 'stellein' (to send). An 'epistle' was something 'sent out' (usually a letter), while 'postal' relates to the sending of mail. This helps us remember that the core meaning involves 'sending'.