The Spanish verb 'postrar' comes from Late Latin 'prostrare' meaning 'to lay flat.' This Latin word was formed from the combination of two elements: the prefix 'pro-' meaning 'forward' and the verb 'sternere' meaning 'to spread or lay flat.' These combined to form the Latin supine 'prostratum,' which then evolved into the Late Latin 'prostrare.' As the word made its way into Spanish, it underwent a slight phonetic change, with the 'r' in 'pro-' being lost, resulting in 'postrar.'

A common related Spanish word is 'postrado' (the past participle of 'postrar'), which is often used in medical contexts to mean 'bedridden' or in religious contexts to mean 'prostrated.' The reflexive form 'postrarse' is particularly common in religious contexts, meaning 'to prostrate oneself' in worship or submission.

The English word 'prostrate' is a direct cognate of Spanish 'postrar,' coming from the same Latin source. While the Spanish dropped the 'r' in the 'pro-' prefix, English maintained it. Both words share the same basic meaning of 'laying flat' or 'lying face down,' whether literally or figuratively. The English word 'prostration' is also related, referring to the act of lying face down, especially in worship, or extreme physical weakness.