Jamieson, Fausset and Brown on Exodus 4:10-13

10-13. I am not eloquent--It is supposed that Moses labored under a natural defect of utterance or had a difficulty in the free and fluent expression of his ideas in the Egyptian language, which he had long disused. This new objection was also overruled, but still Moses, who foresaw the manifold difficulties of the undertaking, was anxious to be freed from the responsibility.