The Spanish word 'tos' (meaning 'cough') comes from the Latin word 'tussis', which also meant 'cough'. The word evolved through Vulgar Latin, where the accusative form 'tussem' was commonly used. Over time, as Latin evolved into Spanish, the word simplified to become 'tos'.

This evolution shows a common pattern in Spanish words derived from Latin, where final syllables often became simplified and some consonants were dropped, making the word shorter and easier to pronounce.

The Spanish verb 'toser' (meaning 'to cough') is directly related to 'tos'. This is a very common pattern in Spanish where nouns and their related verbs share the same root. Other related Spanish words include 'tosidura' (a coughing spell) and 'tosedor' (someone who coughs a lot).

While English 'cough' is not related to Spanish 'tos', there are some English medical terms that share the Latin root 'tuss-', such as 'tussive' (relating to cough), 'pertussis' (whooping cough), and 'antitussive' (cough medicine). These technical terms preserved the original Latin form in English medical vocabulary.