The word 'thou' originates from the Old English 'þū', a singular second-person pronoun. It is directly descended from the Proto-Germanic þū, which in turn derives from the Proto-Indo-European túh₂, the fundamental root for the singular 'you'. This ancient root is remarkably stable, giving rise to cognates across many Indo-European languages, such as Latin 'tū', Greek 'σύ' (sý), and Sanskrit 'tvám'. Over time, English underwent a significant sociolinguistic shift where the plural 'you' began to be used as a formal singular, gradually rendering the intimate or familiar 'thou' obsolete in standard speech, though it persists in religious texts, historic literature, and some regional dialects.

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