What were the Chosam, Pesil and staff that she asked for a security?
Rashi: Chosam was his signet-ring 1 . Pesil was his cloak to cover himself.
Seforno: Pesil was his belt.
Ha'Emek Davar: Pesil was on his outer robe. Esteemed people have it, and very esteemed people have it on the neck. Perhaps it was of Techeles, to remember Kisei ha'Kavod.
What was the significance of the things that she asked for as a security?
Malbim: Normally, a security must be worth as much as what it is in place of; Yehudah did not have this. An esteemed person would not leave his signet ring [or staff] with someone else, so this sufficed.
Ha'Emek Davar: The signet-ring hints to a law-maker. Pesil refers to meditation of Yir'as Shamayim, and the staff hints to a ruler (refer to 38:18:1:2,4).
Why does the Torah add the word "Lo" (after "va'Tahar")?
Rashi: To teach us that the twins with whom she was pregnant were strong like Yehudah and righteous like Yehudah. 1
Ha'Emek Davar: She was indebted to him to establish seed for the deceased.
QUESTIONS ON RASHI
Rashi writes that Pesilecha was his cloak. If so, it should have said "Simlascha"! And surely Yehudah was not naked when he left her!
Refer to 38:18:1:2-4.
Mizrachi: 'His garment to cover himself' hints to the Mitzvah of Tzitzis - "Kesuscha Asher Techaseh Bah." Perhaps it was a small scarf, and he was properly covered even without it. Gur Aryeh - Simlascha would imply all his garments. Pesilecha is only the garment with Tzitzis.