The Spanish word 'muermo' comes from the Latin word 'morbus' (through its accusative form 'morbum'), which meant 'disease'. In Spanish, 'muermo' originally referred specifically to a disease that affects horses (glanders). Over time, the word took on a figurative meaning and is now also used to describe an extremely boring or dull person or thing - perhaps because a sick horse would appear listless and lacking energy.

The Spanish word 'mórbido' (meaning 'morbid' or 'unhealthy') is a related word that comes from the same Latin root 'morbus'. While 'muermo' evolved to refer to a specific horse disease and then to boring things or people, 'mórbido' retained a more general connection to disease and unwholesomeness.

Several English words share the same Latin ancestor as 'muermo'. The most obvious ones are 'morbid' and 'morbus'. While 'muermo' developed to mean a horse disease and later a boring person, the English word 'morbid' retained meanings more closely related to disease, death, and unwholesomeness. The medical term 'morbus' is still sometimes used in English disease names, like 'morbus Parkinson' (Parkinson's disease).