Common - As opposed to holy. (See the marginal references, and compare the use of the word in Acts 10:14-15, Acts 10:28.) It gives an idea of the depressed and poor condition of the priesthood at that time, that Ahimelech should have had no bread at hand except the showbread.
Related Commentaries of 1 Samuel 21:4
1 Samuel 21:4
4 And the priest answered David, and said, There is no common bread under mine hand, but there is hallowed bread; if the young men have kept themselves at least from women.