The Spanish word 'endorfina' comes from French 'endorphine', which was created by combining two elements: the Greek prefix 'endo-' meaning 'within' and the word 'morphine'. This scientific term was coined to describe naturally occurring substances in the body that have effects similar to morphine. The name cleverly reflects that these are morphine-like chemicals produced within the body itself.

Several Spanish words share the Greek prefix 'endo-', such as 'endogamia' (endogamy), 'endoscopio' (endoscope), and 'endocrino' (endocrine). All these words relate to something occurring or existing within something else, just as endorphins are produced within the body.

The English word 'endorphin' is directly related to Spanish 'endorfina', as they share the same French origin. English also uses the prefix 'endo-' in many scientific terms like 'endoscopy', 'endogenous', and 'endothermic'. The 'morphine' part is obviously related to the drug morphine, which comes from Morpheus, the Greek god of dreams, due to the drug's sleep-inducing properties.