The Spanish preposition 'en' meaning 'in' comes directly from the Latin preposition 'in' meaning 'in, into'. This is a straightforward inheritance where both the form and meaning remained quite stable, with just a slight phonetic shift from 'in' to 'en'.

Some common Spanish words that contain 'en' as a prefix include 'entrar' (to enter), 'encima' (on top), and 'entonces' (then). All of these words incorporate the basic meaning of being 'in' or 'into' something.

English speakers will recognize this same Latin root 'in' in numerous English words, as it was preserved in English as both 'in' and 'en-'. We see it in words like 'in', 'into', 'inside', and as the prefix 'en-' in words like 'enclose', 'ensure', and 'entail'. The meaning consistently relates to being inside or moving inward.