The Spanish word 'cojón' meaning 'testicle' comes directly from the Vulgar Latin word 'coleo' which had the same meaning. This is a straightforward inheritance where the Latin word evolved phonetically to its modern Spanish form, maintaining its original anatomical meaning throughout.

Some related Spanish expressions include 'tener cojones' (to have courage/guts) and 'cojonudo' (awesome, excellent), which developed from the anatomical meaning to take on metaphorical meanings related to bravery and impressiveness.

While there aren't direct English cognates from the same Latin root 'coleo', English speakers might find it helpful to know that this word is often used in Spanish expressions similar to how English uses certain anatomical terms metaphorically to express bravery or toughness.