Whenever the noun כף (palm) comes alone, it means the palm of the hand. It always comes alone (except for three places), but כף רגל (palm of the foot) always comes in construct state. And thus, the sages of blessed memory infer that when it says כל הולך על כפיו (whatsoever goes upon its paws)⁶⁹³, this is a monkey.

footnotes: ⁶⁹³ Leviticus 11:27.

And there is a difference between רגל (foot) and כף רגל (palm of the foot): כף רגל indicates the bottom of the foot, while רגל indicates all of it.

Since in Leviticus 11:27,כל הולך על כפיו(whatsoever goes upon its paws), the word כפיו (its paws) comes alone, by Rule 460 it must mean the palms of one’s hands. Then since the only animal that goes upon the palms of its hands is a monkey (its feet are also considered hands, since they can also grab things), it must be talking about a monkey⁶⁹⁴.

footnotes: ⁶⁹⁴ Malbim, HaTorah v’HaMitzvah Shemini 103.