The Spanish word 'laxitud' meaning 'laxity' or 'slackness' comes from the Latin word 'laxus' meaning 'loose' or 'slack'. The Latin word first evolved into the Spanish adjective 'laxo' (meaning 'lax' or 'loose'), and then the noun 'laxitud' was formed by adding the Spanish suffix '-tud', which is used to form abstract nouns indicating a quality or state.

A simpler and more common Spanish word that shares this etymology is 'laxo', meaning 'lax' or 'loose'. This adjective is more frequently used and directly inherited from Latin 'laxus', while 'laxitud' is the abstract noun form describing the state or quality of being lax.

English speakers might recognize the connection to words like 'lax' and 'laxity', which come from the same Latin root 'laxus'. Just as in Spanish, English developed both the adjective form ('lax') and the abstract noun form ('laxity') to describe something loose or slack and the state of being loose, respectively. The English word 'relax' is also related, originally meaning 'to make loose again'.