What is thereof? How to use in English

The word "thereof" is a preposition and a pronoun that originated in Middle English, around the 13th century. Its origin can be traced back to the Old English phrase "thero" or "ðerō," which consisted of the preposition "þær" ("there") and the pronoun "þō" ("thō," which evolved into the modern English word "the"). In the Middle English period, when the English language was transitioning from Old English to Middle English, the two words began to merge together, forming the phrase "thero" or "þerō." This syntax became so common that it eventually evolved into the modern-day word "thereof." During this period, the English language was also heavily influenced by French and Latin, which resulted in the infusion of new words and syntax. This influx of new vocabulary had a significant impact on the evolution of the English language, including the emergence of new words like "thereof." Today, the word "thereof" is considered somewhat archaic, and its usage in written English is quite rare. However, it remains a recognizable part of the English language, standing as a relic of its medieval and Middle English origins.