Today I was trying to “Google” this dictionary, and all I could think of was Jaslow and Jaster, but the name is Jastrow. Maybe google will pick up these corruptions of Jastrow’s name and help someone else in the future: Juster, Jaslo, Jazlo, Jazzlow.
Marcus Jastrow (1829-1903) was a rabbi in Philadelphia. Despite the fact that there have been new discoveries in linguistics since his time, and that he was a Reform rabbi, his work is still widely used, even in Orthodox circles.
His book title: Dictionary of the Targumim, Talmud Bavli, Talmud Yerushalmi and Midrashic Literature
You can find the online/PDF version of the dictionary here:
Link to Dictionary at Tyndale Archive: Jastrow’s Dictionary
The complete work of Marcus Jastrow: Dictionary of the Tagumin, Talmud Bavli, Talmud Yerushalmi, and Midrashic Literature is available online in PDF format. This dictionary includes many Aramaic and Hebrew words that could be called “Rabbinical Hebrew”.