Green’s Dictionary of Slang

Master John Goodfellow n.

also (Master) John Thursday
[generic use of John + SE goodfellow, a jovial companion; John Thursday was a 17C musician and dancing master, the supposed inventor of a dance known as the ‘Hussarde’]

the penis.

[UK]Urquhart (trans.) Gargantua and Pantagruel (1927) I Bk II 357: Here with in, — in showing her his long cod-piece, — Master John Thursday, who will play you such an antic, that you shall feel the sweetness thereof even to the very marrow of your bones. [Ibid.] 360: Hold (showing his long cod-piece), this is master John Goodfellow that asks for lodging.
[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Farmer Vocabula Amatoria (1966) 110: Épervier, m. The penis; ‘Master John Goodfellow’. [Ibid.] 164: Jeudi (Jean). The penis; ‘Master John Thursday’.
[UK]A. Crowley Snowdrops from a Curate’s Garden 37: The German Ambassador [...] had ventured to make his enquiries with his Master John Thursday.
[US]H. Miller Roofs of Paris (1983) 11: I can’t keep her away from my dick, she begins to play with John Thursday.