Green’s Dictionary of Slang

1. a beard that runs along and just beneath the jaw line; when linking up with a moustache it was seen as resembling a door-knocker.

1884		Edinburgh Eve. News 13 Nov. 4/5: His cheeks were shaved; he had a moustache and a ‘door-knocker’ beard encircling his mouth and chin.
1895		Dundee Courier 7 Aug. 3/7: Kent has expressed his disapproval of men [...] with ‘mutton-chop’ whiskers or ‘door-knocker’ appendages on the chin.
1909		(ref. to 1850s) J. Ware Passing Eng. of the Victorian Era 115/2: Door-knocker (Peoples’, 1854). A ring-shaped beard formed by the cheeks and chin being shaved leaving a chain of hair under the chin, and upon each side of mouth-forming with moustache something like a door-knocker.

2. a female hairstyle consisting of two plaits bunched on top of the head.

1868		Liverpool Dly Post 11 Aug. 4/6: Every form of ‘back-hair’ worn by every lady [...] the curly ringlets of the romp [...] the sausage roll, the snake, the caterpillar, the black-pudding, the parasol, the door-knocker and the bird’s nest, all of hair.
1984		Partridge DSUE (8th edn) 331/2: late C.19–early 20.

3. (US black) a large hoop ear-ring.

1995

L. Stavsky et al. A2Z.