The Spanish word 'atrás' meaning 'behind' or 'backwards' comes from combining the preposition 'a-' with 'tras'. The element 'tras' evolved from Latin 'trans' meaning 'across' or 'beyond'. Over time, the Latin 'trans' simplified to Spanish 'tras' and its meaning shifted from indicating movement across something to indicating position or movement behind something.

A simpler related Spanish word is 'tras', meaning 'behind' or 'after'. This is actually the base word that combines with 'a-' to form 'atrás'. You might encounter 'tras' in common phrases like 'tras la puerta' (behind the door).

Several common English words are related to 'atrás' through the Latin root 'trans-'. These include 'trans', 'transit', 'transfer', and 'transport' - all of which maintain the original Latin sense of movement across or beyond something. While the Spanish word evolved to indicate movement backwards or behind, the English words preserved more of the original 'across/beyond' meaning.