John Lightfoot on Mark 7:3

[Except they wash their hands oft.] The fist. When they washed their hands, they washed the fist unto the joining of the arm. The hands are polluted, and made clean unto the joining of the arm. "The Rabbins deliver: The washing of hands as to common things (or common food) was unto the joining of the arm. And the cleansing of hands and feet in the Temple was to the joint." The joining, saith the Aruch, is where the arm is distinguished from the hand. So, also, where the foot is distinguished from the leg.

"The second waters cleanse whatsoever parts of the hands the first waters had washed. But if the first waters had gone above the juncture of the arm, the second waters do not cleanse, because they do not cleanse beyond the juncture. If, therefore, the waters which went above the juncture return upon the hands again, they are unclean."