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.

Today Rashi examines two Hebrew words which can both refer to immersion in water:
• Tet-Beth-Lamed literally means to immerse,
• Kuph-Beth-Samech can mean to immerse and can also refer to cleansing by immersion.

We cite two verses where the nuances of cleaning overshadow the nuance of immersion:
• Verse Gn49-11 states Binding his foal to the vine, and his ass’s colt to the choice vine; he cleans his garments in wine, and his clothes in the blood of grapes;
• Verse Lv15-16 states And if any man’s semen goes out from him, then he shall immerse all his flesh in water, and be unclean until the evening.
Clearly in the first verse the emphasis is on cleaning by immersion. In the second verse, while the word clearly should mean immersion - dipping in a Mikveh body of water - I think there are some nuances that any remaining debris should be cleansed. In fact Jewish law requires showering prior to going to Mivkeh for immersion to remove items clinging to ones body which might prevent full immersion in water. This requirement is hinted at in the use of the word Kuph-Beth-Samech which while meaning immersion also means to clean.

We can invoke another literary rule, FFF, to explain how the word for washing comes to mean to immerse. The FFF rule states that words can be named by Form, Feel and Function. For example the Pentagon is named by its form while the United Nations is named by its function. Similarly hardship is named by its feeling. Applying the triple FFF principle we find that the word for washing can refer to the form of the washing, which is a simple immersion.

Advanced Rashi: Rashi on this verse shows advanced methods. Rashi skillfully takes the Aramaic translation of the word Kuph-Beth-Samech and shows how in some verses the aramaic translation renders this word as immerse while in other verses the aramaic translation renders this word as clean. We however, have chosen a different route. We examined actual verses to show how the two nuances - cleaning and immersion - can blend with each other.