The Spanish word 'agonía' (meaning 'agony') traces its roots back to ancient Greek. It began with the Greek word 'agon' (ἀγών), which meant 'contest' or 'struggle.' This developed into the Greek word 'agonia' (ἀγωνία), which carried the meaning of 'struggle' or 'anguish.' The word was then borrowed into Latin as 'agonia,' maintaining similar meanings of struggle and anguish, before finally entering Spanish as 'agonía.'

The semantic development from 'contest/struggle' to 'agony' makes intuitive sense - what was originally a term for a physical contest or struggle gradually came to represent the mental and physical suffering that might accompany such a struggle.

A related Spanish word is 'agonizar' (to be in agony, to be dying), which is the verb form derived from the same root. Another related term is 'agonista,' which in modern Spanish can refer to a protagonist or main character - reflecting the original Greek sense of someone engaged in a contest or struggle.

The English word 'agony' is a direct cognate of Spanish 'agonía,' as they share the same Greek ancestor. Other related English words include 'antagonist' (someone who struggles against another, from 'anti-' + 'agon'), and 'protagonist' (the main character who struggles in a story, from 'protos-' meaning first + 'agon'). Even the word 'agenda' is distantly related, as it developed from the idea of things that must be struggled with or accomplished.