take down a notch

Also, take down a peg. Deflate or humble someone, as in He's so arrogant that I wish someone would take him down a notch, or That defeat took them down a peg. Both notch and peg in this idiom allude to a series, the former of indentations, the latter of knobs, used to raise or lower something. Specifically, peg alludes to the pegs used to lower a ship's colors. Their figurative use dates from the second half of the 1600s. Also see cut down, def. 4.