When Rashi uses the synonym method he does not explain the meaning of a word but rather the distinction between two similar words both of whose meanings we already know.

The following Hebrew words all refer to suddenness.
• Pay-Tauv-Aleph-Vav-Mem, Pithom, suddenly;
• Pay-Tauv-Ayin, PeTha unexpectedly.

In our article Peshat and Derash: A New Intuitive and Logical Approach, which can be found on the world-wide-web at http://www.Rashiyomi.com/rashi.pdf we have advocated punchy translations of Biblical verses as a means of presenting Rashi comments. The following translation of verse Nu35-22 embeds the Rashi translation Petha means unexpectedly. But if he thrust him unexpectedly without enmity, or hurled upon him any thing without lying in wait,

Advanced Rashi: We have translated the words Pithom and Petha as meaning suddenly and unexpectedly based on context. This may also be partially supported by the resemblance of the Pay-Tauv-Ayin and Pay-Tauv-Cheth roots. Pay-Tauv-Cheth means Door. So unexpectedly in Hebrew means from behind the door; a concept indicating nearby and present but unseen so it is unexpected. There is a certain amount of conjecture in our translations since there are very few verses with either of the words and the etymologies although possibly supportive don't prove anything.

More importantly our translations mirror Jewish law.
• A non-sudden attack doesn't always carry a death penalty. For example if I throw a knife, with intent to kill, at a person with a shield, then I am exempt from a death penalty if the shield is removed (even if I am the one removing it). The point here is that the act of murder is not immediate since the knife must fly through the air and penetrate the person and therefore intermediate action (such as shield) may prevent death. Needless to say an act of murder has been committed; however it is not punishable by death. However in some situations, e.g. throwing a knife at a person without a shield, non-suddenness does not exempt from a death penalty.
• An unexpected attack doesn't always carry a death penalty. For example, if I threw a knife, and after I threw it someone came out from behind the door (Pethach = door = behind the door = unexpected) then the act is neither considered willful nor negligent.

The above interactions between the translations and details in Jewish law afford us a rare glimpse at the interaction of language and law.