Bereishis Rabbah 80:11, on the identification of the foreskin as “cherpah.”
Sefaria · Bereishis Rabbah 80:11
כי חרפה — רבי נחמיה אמר: הֵיכָן מָצִינוּ שֶׁנִּקְרֵאת עָרְלָה חֶרְפָּה? מִן הָכָא, שֶׁנֶּאֱמַר: כִּי חֶרְפָּה הִוא.
“For that is a disgrace” — Rabbi Nechemiah asks where we find that the foreskin is called a disgrace, and answers: here.
The foreskin is characterized as a cherpah, a source of disgrace, in the brothers’ negotiation.