The Spanish word 'arcoiris' (rainbow) is a compound word formed from two parts: 'arco' (arch, bow) and 'iris' (rainbow). The first element 'arco' comes from Latin 'arcus', which meant 'bow' or 'arch'. The second element 'iris' traces back to Ancient Greek 'ἶρις' (iris), meaning 'rainbow' or 'iris of the eye', which entered Spanish through Latin 'iris'. The compound literally means 'bow of rainbow' or 'rainbow arch', describing the characteristic curved shape of this natural phenomenon.

The Spanish word 'arco' by itself is a common word meaning 'arch' or 'bow', which you might see in architectural terms like 'arco romano' (Roman arch) or in sports equipment like 'arco de fútbol' (soccer goal). The word 'iris' also exists independently in Spanish, referring to both the rainbow and the colored part of the eye, just as it does in English.

English speakers can easily remember this word by connecting it to familiar English words. 'Arch' comes from the same Latin root 'arcus' as Spanish 'arco'. The word 'iris' exists in English with the same meanings as in Spanish - both for the part of the eye and in reference to the rainbow (as in 'iridescent', meaning showing rainbow-like colors). The English word 'arc' is also related, describing any curved line or path, just like the shape of a rainbow.