The Spanish word 'morcillo' has two distinct etymological origins and meanings. In its anatomical sense meaning 'muscle', it comes from Latin 'musculus' meaning 'little mouse; muscle', which itself is a diminutive of Latin 'mus' meaning 'mouse'. The connection between mice and muscles comes from the ancient observation that certain muscles, when flexed, appeared to move under the skin like a little mouse. The Latin word evolved into archaic Spanish 'murecillo' before becoming modern 'morcillo'.

In its second meaning describing a 'reddish-black' color (particularly of horses), 'morcillo' comes from Latin 'mauricellus' meaning 'little Moor', a diminutive of 'maurus' meaning 'Moor' or 'dark-skinned'. This etymology reflects historical color terminology derived from ethnic references.

The Spanish word 'músculo' (muscle) is a more common related word that shares the same Latin root 'musculus' with the anatomical sense of 'morcillo'. While 'músculo' is the modern general term for muscle, 'morcillo' has become more specialized in anatomical contexts.

For the color meaning, the Spanish word 'moro' (Moor) is related to 'morcillo', both ultimately coming from Latin 'maurus'.

The English word 'muscle' is closely related to the anatomical sense of 'morcillo', as both derive from Latin 'musculus'. The English word preserved more of the original Latin form, while the Spanish word underwent more significant sound changes.

The English word 'Moor' is related to the color sense of 'morcillo', both coming from Latin 'maurus'. This connection is preserved in English words like 'moorland' and 'mooring', originally referring to dark or black places.