Rashi employs the methods of
idiom is a group of words whose meaning
transcends the meanings of the
individual units. Most people think of the
dictionary as listing meanings of individual words.
However a dictionary should also list the meanings of
idioms, groups of words.
Verse Ex21-20:21 should be translated as follows
And if a man smite his bondman, or his bondwoman,
with a rod, and he die under his hand,
he shall surely be punished. Notwithstanding
if he survives for 24 hours he shall not be punished;
for he is his money.
The above verses states that there is a
death penalty for killing one's slave provided the
slave dies within 24 hours.
The actual Biblical verse does not use the term
24 hours. Rather it uses the literal term
day or days. Rashi identifies this as an idiom:
The Biblical day or days is an
idiom meaning 24 hours.
The etymology of the idiom is a uniform period that
corresponds to a day but can span two days.