Green’s Dictionary of Slang

1. to lose impetus, to lose enthusiasm.

[1904		Shields Dly Gaz. 9 Apr. 4/5: The Chairman [...] said that in the absence of Mr Chamberlain the fiscal kettle seemed to have gone off the bvoil — (laughter)].
1908		Aberdeen Jrnl 5 Sept. 4/6: He felt the enormity of their kindness had taken the wind out of his sails and the water had gone off the boil and the ship was running dead slow.
1914		Iron Age 94 795: Finished stel is rather quiet [...] There is really no alteration fundamentally, but the tone has gone off the boil and prices have shed a few shillings.
1915		Dundee, Perth [etc.] People’s Jrnl 25 Dec. 10/1: His hate for this country ahs never gone off the boil.
1920		‘Sapper’ Bulldog Drummond 165: He doesn’t strike me as being Number One value. He’s gone off the boil.
1934		Wodehouse Right Ho, Jeeves 106: It was, of course, the shark [...] that had caused love’s young dream to go temporarily off the boil.
1938		Dunde Courier 5 Dec. 9/3: Wolverhampton are having a grand spell. They beat Preston, who seem to have gone ‘off the boil’.
1944		Sun. Post (Lanarks) 19 Nov. 2/3: Shipping shares have for the present gone off the boil, but Cundards held 21s 9d.
1963		H.E. Bates Oh! To be in England (1985) 349: You’ve been a bit quiet lately. I’d almost begun to think you’d gone off the boil.
2000		Guardian Guide 22–28 Jan. 25: He was one of the first world-class virtuosi of the British jazz scene [...] and has never gone off the boil in the last three decades.

2. of a woman, to lose her enthusiasm for sex.

1980		Maledicta IV:2 (Winter) 197: On the other hand, she may go off the boil and may then employ the freeze to get what she wants.
1984		Partridge DSUE (8th edn) 476/2: Aus. since ca. 1920.