The Spanish word 'ultimátum' comes from the Latin word 'ultimatum' meaning 'final demand'. This Latin word was derived from the verb 'ultimare' meaning 'to come to an end', which in turn came from the adjective 'ultimus' meaning 'farthest' or 'last'. Going further back, 'ultimus' developed from 'ulter' meaning 'that is beyond', which originated from the Latin root 'uls' meaning 'beyond'. The word maintains essentially the same meaning today in Spanish as it had in Latin - a final demand or statement of terms, the rejection of which will result in a specific consequence, often hostile action.

Some related Spanish words that might be more familiar to Spanish learners include 'último' (last, final), 'ulterior' (ulterior, further), and 'ultra-' (ultra-, beyond, extreme). All these words share the same Latin ancestry and carry meanings related to being last, final, or beyond.

English speakers will recognize the cognate 'ultimatum', which has the same meaning and spelling (minus the accent mark). Other related English words include 'ultimate' (final, fundamental), 'ulterior' (as in 'ulterior motive'), and words with the prefix 'ultra-' meaning 'beyond' or 'extreme' (like 'ultraviolet' or 'ultra-modern'). All these words can be traced back to the same Latin roots meaning 'beyond' or 'last'.