The Spanish word 'sólo' (meaning 'only' when used as an adverb) comes from the Latin word 'solus' meaning 'alone'. It first entered Spanish as the adjective 'solo' (meaning 'alone'), and then developed the adverbial form 'sólo' with a slight shift in meaning to 'only'. The accent mark was added to distinguish the adverb 'sólo' from the adjective 'solo', though in modern Spanish this accent is now optional according to current spelling rules.

The Spanish adjective 'solo' (meaning 'alone' or 'single') is directly related to 'sólo', as they both come from the same Latin root 'solus'. While 'solo' retained the original meaning of 'alone' as an adjective (as in 'un hombre solo' - 'a lonely man'), 'sólo' developed into an adverb meaning 'only' or 'just' (as in 'sólo quiero uno' - 'I only want one').

Several English words share the same Latin ancestor 'solus'. The most obvious is 'solo', which we use both as a noun and adjective, particularly in music and performance (like 'a solo performance'). Other related English words include 'solitude' (the state of being alone), 'solitary' (alone, single), and 'desolate' (abandoned, lonely). All these words carry the core meaning of 'alone' or 'single' from their Latin origin.