The Spanish verb 'acostar' (to lay down) comes from combining the Latin prefix 'a-' (meaning 'to, towards') with the Latin word 'costa' (meaning 'rib, side'), plus the Spanish infinitive verb suffix '-ar'. The etymology reflects the physical action of laying down - literally moving towards one's side. This makes intuitive sense since when we lie down, we often rest on our side.

Some related Spanish words that might help you remember 'acostar' include 'costa' (coast, shore) and 'costilla' (rib). These words share the Latin root 'costa'. Just as a coast is the side of a land mass where it meets the sea, and ribs are the bones on the sides of your chest, 'acostar' relates to putting yourself on your side.

English speakers might recognize the connection to words like 'coastal' and 'coast', which come from the same Latin root 'costa'. While these English words maintained the connection to the seaside or edge meaning, the Spanish 'acostar' developed to specifically refer to laying down on one's side.