Green’s Dictionary of Slang

1. (Aus.) to aim a blow (at); cite 1849 suggests firing a pistol.

1849		Leamington Spa Courier 4 Aug. 4/5: They have the pops ready — and the Setters are out, and they are determined to let out the stash.
1885		Bulletin (Sydney) 31 Oct. 22/1: His temper was up, and he had to let out at something. Like the Irishman who, when offended by his wife, fell upon an omadhaun in the kitchen and gave him the ‘grandfather av a batin’,’ Joe, unable to vent his chargrin on any suitable object, took it out of the young ’uns.
1899		Eve. Teleg. (Dundee) 30 Aug. 4/3: ‘You may think me a soft mark [...] but I’ll show you that I’m not,’ and with these words he let out at his assailant.
1905		Bulletin (Sydney) 3 Aug. 12/2: Percy let out with his left. A bobby grabbed it. He swung in the right. Another secured it.
1937		A. McLean Vive la Legion 101: ‘Some blasted Frog insulted us, so I let out.’ ‘Kill ‘im?’ he asked. ‘God in heaven, no,’ I replied. ‘I only hit him ’.

2. to admonish.

1899

R. Whiteing No. 5 John Street 41: I heard that old cure lettin’ out at the aristocracy arter I had floored the bloke.