Thorn in the Flesh.
This was something that Satan was allowed to inflict on Paul, not because of any fault he had committed, but lest he should be puffed up on account of his having been caught up into the third heaven. 2 Corinthians 12:7-10. Apparently it was something that made him contemptible in the eyes of his fleshly opponents. See 2 Corinthians 10:10; 2 Corinthians 1:11; Galatians 4:13, Galatians 1:4.