The Name of the Angel

"What is your name," Yaakov asked the angel with whom he had wrestled. Why didn't Yaakov know the angel's name? Isn't it widely known that the Angel of Esav, the epitome of evil, is named סמא-ל (Samuhl). The Ishbitzer Rebbe explains that the first two letters of the name of the angel סם are the only two letters in the Hebrew aleph-bet that are completely closed. This indicates that the spiritual power inherent in these letters is bottled up and hidden from man's understanding. Even though the last two letters of the angel's name form the name of God, indicating that there is great holiness within סמא-ל as there is in all angels, the essence of this angel is a power that man cannot reach.

This is alluded to in the Talmud when it says that the letters סם in the Luchos (the tablets brought by Moshe) stand miraculously. The idea is that whereas all the others letters desire to share their power with man, it is only by an occasional miracle that man can truly comprehend the holiness inherent in these two letters.
Yaakov was asking for the name of the angel. A name is a revelation of the essence of the one carrying the name. That was something which סמא-ל could not share with Yaakov. His true essence would remain hidden.
This was the same angel who later tried to dissuade God from giving the Torah to the Jewish nation. True to his essence of standing for not revealing deeper truths to others, he felt that the secrets of the Torah did not belong to man. He hit Yaakov in the גיד הנשה (perhaps the sciatic nerve). The Talmud teaches us that אין בגידים בנותן טעם this part of the body has no taste. He was trying to tell Yaakov that as a person he cannot hope to get any taste, any true depth of understanding.
But Yaakov fought the angel and persevered.