Albert Barnes Commentary Deuteronomy 25:3

"Forty stripes he may give him, he shall not exceed; lest, if he should exceed, and beat him above these with many stripes, then thy brother should seem vile unto thee." — Deuteronomy 25:3 (ASV)

The Jews, to keep within the letter of the law, fixed 39 stripes as the maximum (compare the marginal reference). Forty signifies the full measure of judgment (Numbers 14:33–34); but the son of Israel was not to be lashed like a slave at the mercy of another. The judge was always to be present to see that the Law in this particular was not overpassed.