Green’s Dictionary of Slang

peg n.2

1. a blow, esp. a straight-armed jab; thus peg in the daylight, a blow in the eye; peg in the victualling office, a blow in the stomach; peg in the haltering place, a blow under the ear.

[UK] in J.P. Collier Hist. Eng. Dramatic Poetry (1831) 198: Strike a pegge into him with a club [OED].
[UK]Smollett Roderick Random (1979) 155: Many cross-buttocks did I sustain, and pegs on the stomach without number, till at last my breath being quite gone, as well as my vigour wasted, I grew desperate.
[UK]Smollett Peregrine Pickle (1964) 745: Pipes [...] slyly bestowed upon him a peg under the fifth rib.
[UK]Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Peg, a blow with a Streight Arm, a term used by the professors of the Gymnastic Art. A Peg in the Jaws, the Victualling Office or Haltering place, a Blow in the face, stomach or under the Ear.
[UK]Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: A peg is also a blow with a straight arm: a term used by the professors of gymnastic arts. A peg in the day-light, the victualling office, or the haltering-place; a blow in the eye, stomach, or under the ear.
[UK]Egan Boxiana I 25: And, with one ‘English peg’ in the stomach (quite a new thing to foreigners) brought him on his breeech.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.

2. in fig. use, a metaphorical blow, a verbal attack.

[UK]C. Rook Hooligan Nights 81: All of ’em put in a peg for me as well as they knew ’ow.