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

Verse Lv19-15a states Ye shall not be red-taped in judgment; thou shalt not respect the person of the poor, nor favour the person of the mighty; but in righteousness shalt thou judge thy neighbour. We have translated the Hebrew word, Ayin-Vav-Lamed, as meaning red-taped. The root in question means burden. A burdensome judicial process, is a red-taped judicial process - a judicial process where you are burdensome (but not necessarily unjust) to the parties seeking judicial guidance.

The word red-taped is not perjorative. Rashi wished to inject an element of perjorativeness in this word. Rashi accomplishes this by finding three synonyms:
• Tauv-Vav-Ayin-Beth-Hey which means abominable,
• Shin-Kuph-Tzade which means a rat/vermin/rodent/low life (Similar metaphors in English!)
• Cheth-Resh-Mem which means excommunicated
By bringing these synonyms Rashi shows that the driving connotation of the entire class of words - burdensome, red-taped, rat-like, excommunicated - is perjorative and indicates something despicable. The individual synonyms indicate how the despicableness is implemented.

Advanced Rashi: Rashi actually is more crafty. He skillfully cites several verses, each verse with a pair of these four synonyms linked, thereby showing that the explicitly perjorative connotations of some of these words are transferred to the lighter words. We however have sufficed with showing Rashi's basic approach of classifying the given word in a larger class of synonyms all of which refer to something bad.