"insane, eccentric," British slang, by 1917 in the armed services and in full doolally tap (with Urdu word for "fever"), from Deolali, near Bombay, India, which was a military camp (established 1861) with a large barracks and a chief staging point for British troops on their way to or from India; the reference is to men whose enlistments had expired who waited there impatiently for transport home.