"I may be unskilled in speech." With these words, Paul may simply be comparing himself with other speakers who were better educated in Greek eloquence, or indicating that he could not quite live up to the expectations of some of his readers who have a very high standard of Greek oratory and eloquence.
Paul's affirmation here, however, may be related to the severe illness which he mentions in Galatians 4:13 and which he implies, affected him so badly that he was even repulsive to look at.
There are two possible suggestions about the effect of that illness, one concerning his sight, the other concerning his speech.
We will never know what happened, but we do know that his disability did not interfere with his apostolic work. Here there is a great inspiration for all of us who are disabled in any way, whether by illness or advancing age or both. We now know that a disability need not be a handicap, for our greatness lies not in great achievements, but in being beloved children of God and sincere servants of Christ's mission, as Saint Paul and so many others have been.
Father in Heaven, as You have given us the gift of faith, grant that our weaknesses, faults and failings may never be an obstacle to the Gospel.