The Spanish possessive adjective 'su' (meaning 'his', 'her', 'its', or 'their') comes from the Latin word 'suus', which meant 'his/her/its own'. This evolved into Old Spanish 'súo', which then developed into the modern Spanish possessive pronoun 'suyo'. The shorter form 'su' developed as the unstressed possessive adjective form that comes before nouns, while 'suyo' remains as the stressed pronoun form used after verbs or at the end of phrases.

The most closely related Spanish word is 'suyo', which is the possessive pronoun form of 'su'. While 'su' is used before nouns (like in 'su casa' - his/her house), 'suyo' is used after verbs or at the end of phrases (like in 'es suyo' - it is his/hers). Both words express the same concept of possession but are used in different grammatical contexts.

While there aren't any common English words directly related to 'su' through the Latin 'suus', English expresses the same concept through the possessive pronouns 'his', 'her', 'its', and 'their'. Unlike Spanish, which uses the same word 'su' for all these meanings, English has distinct forms for each person and gender.