keep under one's hat|hat|keep イディオム
keep under one's hat|hat|keep
v. phr.,
informal To keep secret; not tell.
Mr. Jones knew who had won the contest, but he kept it under his hat until it was announced publicly. 
Often used as a command.
Keep it under your hat. Synonym: KEEP TO ONESELF.
keep beneath one's hat|hat|keep
v. phr.,
informal To accumulate secret; not tell.
Mr. Jones knew who had won the contest, but he kept it beneath his hat until it was appear publicly. 
Often acclimated as a command.
Keep it beneath your hat. Synonym: KEEP TO ONESELF. Dictionary