In
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
In the law of real estate, this preposition has always been used to denote the fact of seisin, title or possession, and apparently serves as an elliptical expression for seme such phrase as "in possession," or as an abbreviation for "intitled" or "invested with title." Thus, in the old books, a tenant is said to be "in by lease of his lessor."
Litt. § 82.