Keep it under your hat is an admonishment to keep something secret. Definition of under hat in the Idioms Dictionary. What does under hat expression mean? UNDER YOUR HAT was enough of a hit to lend a semblance of its title to Hulbert & Courtneidge's postwar stage hit based on the black market, UNDER THE COUNTER, which played over a year in London and another in OZ (but barely held out a month on Broadway separated as it was by a profoundly different wartime experience and five years of OKLAHOMA's progeny). Over the years, the idiom keep it under your hat seems to have undergone a slight shift in meaning. Definitions by the largest Idiom Dictionary. What does under hat expression mean? In the mid-1800s the term was used in Britain as an admonishment to keep something in your head, to leave something in your imagination and not bring it to fruition. Definitions by the largest Idiom Dictionary. under hat phrase.