The Spanish word 'solo' comes directly from the Latin word 'solus', meaning 'alone' or 'only'. The word maintained both its form and meaning quite closely as it evolved from Latin to Spanish, with just a minor change from 'solus' to 'solo'.

Some related Spanish words include 'solamente' (only, just), 'solitario' (solitary, lonely), and 'soledad' (solitude, loneliness). All these words share the same Latin root 'solus' and relate to concepts of being alone or singular.

English speakers can easily remember 'solo' by connecting it to several similar English words that come from the same Latin root: 'solo' (as in a solo performance), 'solitary', 'solitude', and 'sole'. All these English words share the core meaning of being alone or unique. The musical term 'solo' is actually borrowed directly from Italian, which like Spanish, inherited it from Latin 'solus'.