Exodus 21:18

And when men strive together, and one smite his neighbour with a stone, or with his fist, so that he die not, but fall ill,

WITH A STONE OR ‘B’EGROF’ (WITH HIS FIST). Judging from certain expressions of the Rabbis, egrof is the hand closed tightly, with the fingers gathered into the palm for the purpose of striking someone with it. Thus they speak of ba’alei egrofin [literally: “men of fists,” or men of power]; “egrofo (the fist) of Ben Abtiach.” Similarly ‘b’egrof’ of wickedness means the hand of wickedness that strikes. Scripture thus mentioned two kinds of hitting — the hard one, with a stone, and the lighter one, with the fist, which in most cases does not cause death — in order to declare that in both cases an estimate [of the power of the stone or the fist with which he struck him] is needed, and the assailant is meanwhile imprisoned. If the assailed person dies, such as where he struck him a mortal blow, he is a murderer; he shall surely be put to death, but if he does not die, the assailant has to pay him for the loss of his labor and for the cost of the healing.
In the Mechilta [we find the following text]: “Rabbi Nathan says: He has likened the stone to the fist and the fist to the stone. Just as the stone can cause death, so also the blow of the fist must be such that it is capable of causing death. And just as the fist is something that can be identified [and measured for the power of its blow], so also the stone must be such as can be identified. But if the stone with which he struck him became mixed up with other stones, the assailant is free.” This means if the stone became mixed up with other stones, they [e.i., the judges] estimate the lightest one and if it is incapable of causing death, they free the assailant.
Other scholars). have said that the word egrof means “a clod of earth,” the term being derived from the expression: the grains shrivel under ‘megr’fotheihem’ (their clods of earth), and the letter alef [in the word egrof is redundant], as is the alef in the word ezro’a [“arm,” where the word is really zro’a]. And this is the opinion of Onkelos who translated b’egrof — b’churmeiza (a piece of white stone or glazed tile). He mentioned these things also in order to tell us that the assailant who hits him with a clod of earth is also liable to be put to death, if he struck him on a sensitive spot where it is capable of causing death. This is to distinguish the law of the sword from the stone and egrof, for no estimate [of a sword or weapon] needs to be taken [since even a minute piece of metal — such as a needle — can also cause death], as the Sages have mentioned.

או באגרוף, according to the plain meaning, as Onkelos translates, i.e. with a stone or brick. This is also the opinion of Torat Kohanim in its commentary on Leviticus 14,40. The departure from understanding the word אגרוף as “fist,” is justified with the question: “is someone to be executed for hitting merely with a fist, not a lethal tool?” Isaiah 58,4 להכות באגרוף רשע, “to strike wickedness with a fist” would hardly be a deterrent. It is therefore understood to refer to a large stone. Moreover, when the Torah reports that the victim did not die, surely one does not normally die by being struck with a fist; it stands to reason that the Torah refers to a lethal weapon which failed to kill, i.e. a brick or something similar held in one’s fist causing serious injury.