Which philosopher is most well known for theorizing that the mind at birth is tabula rasa or a "blank slate"?

It was "John Lock" who theorized it in 17th century.

Tabula rasa is a Latin expression, as mentioned in the question it means "clean slate" in epistemology and brain research, an assumed condition that empiricists qualities to the human personality before thoughts have been engraved on it by the response of the faculties to the outer universe of items.