The Spanish pronoun 'tú' meaning 'you (singular)' comes directly from the Latin pronoun 'tu', which also meant 'you' in the nominative case. This is one of the most stable pronouns in the Indo-European family, maintaining its basic form and meaning over thousands of years with very little change. The only modification from Latin to Spanish was the addition of the accent mark to indicate stress.

The pronoun 'tú' is one of the most basic and essential words in Spanish. It's related to other forms of the second person singular like 'te' (object pronoun) and 'ti' (prepositional pronoun), as well as the possessive 'tu' (your) - notice this last one has no accent mark.

While English 'you' comes from a different source, English speakers might be interested to know that the Latin 'tu' is actually related to English 'thou', which was historically the informal singular form of 'you' in English. We can also see this same ancient pronoun root in the German 'du' and French 'tu', showing how widespread and stable this pronoun has been across Indo-European languages.