The Spanish word 'opaco' (meaning 'opaque') comes from the Latin word 'opacus' meaning 'dark' or 'shaded'. The Latin word was formed by combining the prefix 'ob-' (meaning 'against' or 'toward') with the adjectival suffix '-acus'. The prefix 'ob-' suggests the idea of something standing against or blocking, which evolved into the modern meaning of being impenetrable to light or not transparent.

Some related Spanish words include 'opacidad' (opacity) and 'opacar' (to make opaque or to overshadow). These words share the same Latin root and maintain similar meanings related to blocking light or diminishing transparency.

The English word 'opaque' is a direct cognate of Spanish 'opaco', as both derive from Latin 'opacus'. The English word maintains the same meaning of being neither transparent nor translucent. Other related English words include 'opacity' and 'opacify'. The similarity between the Spanish and English words makes this an easy vocabulary item for English speakers to remember.