Green’s Dictionary of Slang

the penis.

1883		‘Three Chums’ in Boudoir II 46: ‘J. T. is quite himself again,’ exclaimed Charlie, as he threw off the bed-clothes to survey the grand proportions of that part of his anatomy sacred to the service of the fair sex.
1980		J. McClure Spike Island (1981) 140: Half-fill a jam jar with dettol and then top with warm water. Just put yer JT in it and let it soak for half an hour.