The Spanish word 'mejor' (meaning 'better') comes from the Latin word 'melior', which also meant 'better'. 'Melior' was formed with the Latin comparative suffix '-ior', which was used to create comparative forms of adjectives. During the evolution from Latin to Spanish, 'melior' underwent a phonetic change where the 'l' changed to a 'j' sound, resulting in 'mejor'.

'Mejor' is already one of the most basic and common Spanish words, being an irregular comparative form of 'bueno' (good). You'll find it in common phrases like 'el mejor' (the best) and 'lo mejor' (the best thing).

While English 'better' is not related to Spanish 'mejor', there are some English words that share the Latin root 'melior'. These include 'ameliorate' (to make better), 'meliorate' (to improve), and 'meliorism' (the belief that the world tends to improve). However, these are rather formal or academic words that aren't commonly used in everyday English.