The Spanish possessive adjective 'mi' (meaning 'my') comes from the Latin word 'meus', also meaning 'my'. It developed through an intermediate form 'mío', which is still used in Spanish today as a possessive pronoun. The shorter form 'mi' emerged as the unstressed possessive adjective that comes before nouns (as in 'mi casa' - my house), while 'mío' is used predicatively or after the noun (as in 'es mío' - it is mine).

The most closely related Spanish word is 'mío', which is the stressed possessive pronoun form meaning 'mine'. While 'mi' is used before nouns ('mi libro' - my book), 'mío' is used after the verb ser ('es mío' - it is mine) or after nouns with an article ('un amigo mío' - a friend of mine).

While not directly related to the Spanish 'mi', English speakers might find it interesting that the English words 'me', 'my', and 'mine' share a similar ancient origin in Proto-Indo-European. All these words relate to the concept of personal possession and the first person singular, though they developed differently in Germanic languages (like English) versus Romance languages (like Spanish).