The Spanish word 'acostado' (meaning 'laid down') comes from the verb 'acostar' ('to lay down'), which itself is formed from three parts: the verbal prefix 'a-', the Latin root 'costa' meaning 'rib' or 'side', and the Spanish infinitive suffix '-ar'. The past participle suffix '-ado' was added to form 'acostado'.

The semantic evolution is quite intuitive: from the Latin word for 'rib/side', the verb developed to mean 'to place on one's side' or 'to lay down', as when someone lies down to rest, they typically lie on their side.

Some related Spanish words that might be familiar to Spanish learners include 'costa' (coast, shore) and 'costado' (side, flank). These words share the same Latin root 'costa'. While 'acostado' refers to being laid down, 'costa' retained a meaning related to the 'side' of something - in this case, the side where land meets the sea. 'Costado' similarly kept the basic meaning of 'side'.

English speakers might recognize the connection to words like 'coastal' and 'coast', which come from the same Latin root 'costa'. While the Spanish 'acostado' developed from the idea of laying on one's side, the English words developed from the concept of the 'side' of land that meets the sea. Another related English word is 'costal', which is used in medical terminology to refer to things relating to the ribs, maintaining the original Latin meaning.