Albert Barnes Commentary 1 Samuel 21:4

"And the priest answered David, and said, There is no common bread under my hand, but there is holy bread; if only the young men have kept themselves from women." — 1 Samuel 21:4 (ASV)

Common—as opposed to holy. (See the marginal references and compare the use of the word in Acts 10:14–15 and Acts 10:28.) This gives an idea of the depressed and poor condition of the priesthood at that time, since Ahimelech had no bread at hand except the showbread.