Urquhart, in translating Rabelais, often uses this term for buttocks, anus. I thought it high time that I checked the etymology.
nockandro, n. < NOCK n.1 + -andro, of uncertain origin, but perh. < ancient Greek andr-, man (see ANDRO- comb. form), in order to distinguish it from a woman’s nock.
1611 R. COTGRAVE Dict. French & Eng. Tongues, Cul,..[a] tayle, nockandroe, fundament. 1653 T. URQUHART tr. Rabelais Wks. II. xix. 139 Panurge put one finger of his left hand in his nockandrow.
Nock, 2a The cleft in the buttocks, the anus. (Also 2b, slang. A woman’s external genitals; the vulva.) Originally “either of the small tips of horn fixed at each end of a bow and provided with a notch for holding the string. Later: such a notch cut in this or in the bow itself”. The origin is ‘uncertain’; of course, one thinks instantly of notch, but OED says, “ This word corresponds closely in form and sense with both NICK n.1 and NOTCH n., although no etymological relationship with either word has been established.”
Lovely word, and to my delight OED has a modern citation
1993 Los Angeles Times (Nexis) 16 July E1/2 Madonna is well-known for her muscular nockandro and most other parts of her anatomy.
However I fancy that it’s being used as a conscious archaism there. Googling provides no evidence of general usage.