Green’s Dictionary of Slang

crazy, insane; usu. as go fanti, to go crazy, to run amok.

1885		Era (London) 18 July 8/3: Numerous instances are on record of savgses who, brought to England and broken into the broad-cloth and prayer- meetings of civilization, have revisited their native shores as missionaries only to fling off the white choker and revel once more in the barbaric orgies of their former tribe. They are said to ‘go Fanti.’ So Theodora has these occasional fits of going Fanti.
1891		Kipling Departmental Ditties 59: ‘Went Fantee’—joined the people of the land. / Turned three parts Mussulman and one Hindu.
1896		R. Devereux Ascent of Woman 124: Only to take it out of its hand-box seemed to induce a tendency to ‘go fanti.’ [...] I gave it away to an enemy who was going abroad.
1929		(con. 1916) F. Manning Her Privates We (1986) 29: When he got on his feet again, he went abso-bloody-lutely fanti.
1930		G.K. Chesterton Four Faultless Felons 190: He was a white man, or a whitish man, who had gone fantee and wore nothing but a pair of spectacles and and worshipped a god of his own that he had made out of an old umbrella.
1930		(con. 1914–18) Brophy & Partridge Songs and Sl. of the British Soldier.