The Spanish verb 'opacar' (meaning 'to make opaque') comes from the Spanish adjective 'opaco' ('opaque') combined with the verbal suffix '-ar'. The adjective 'opaco' itself derives from the Latin word 'opacus', meaning 'dark' or 'shaded'. This development shows how the Latin term for darkness or shade evolved into the Spanish concept of making something non-transparent or dim.

A simpler and more common related Spanish word is the adjective 'opaco' (meaning 'opaque' or 'dull'). While 'opaco' describes the state of being non-transparent or dim, 'opacar' is the action verb form that means to make something take on that quality.

The English word 'opaque' is a close relative of 'opacar', as both trace back to the Latin 'opacus'. While Spanish turned this into a verb meaning 'to make opaque', English maintained it primarily as an adjective meaning 'not transparent' or 'difficult to understand'. The similarity in both spelling and meaning makes this connection easy to remember for English speakers learning Spanish.