Offscouring means refuse, filth, scrapings, or something treated with contempt. It is a strong expression for people reduced to disgrace, humiliation, and social rejection.
Jeremiah uses the word for Jerusalem, once fair and holy, after it was despoiled and derided by enemies. Paul applies it to the apostles, who were treated as the filth or offscouring of the world for Christ’s sake.
The image conveys the humiliation of a people whose city lay in ruins and whose survivors were subjected to contempt and slavery. In the apostolic setting it expresses the disgrace endured before an unbelieving world and false apostles.