The Spanish word 'rectal' comes from combining two Latin elements: 'rectus' meaning 'straight', and the suffix '-al' meaning 'relating to'. The Latin 'rectus' itself derives from the verb 'regere' meaning 'to straighten' or 'to guide'. The word evolved through Spanish 'recto' (straight) before taking on its anatomical meaning with the addition of the '-al' suffix.

Some simpler related Spanish words include 'recto' (straight), 'rectitud' (rectitude, righteousness), and 'rectificar' (to rectify, correct). All these words share the basic notion of 'straightness' or 'correctness' from Latin 'rectus', though they've developed different specific uses.

Several English words share this Latin ancestry, including 'rectify' (to make right), 'rectangle' (a shape with straight angles), 'direct' (to guide straight), and 'correct' (right, proper). The English word 'rectal' is actually a direct cognate of the Spanish word. All these terms trace back to the Latin root meaning 'straight' or 'right'.