idle/empty threat

:  lacking worth or basis :  vain

:  not occupied or employed: such as

:  having no employment :  inactive

:  to spend time in idleness

:  to move idly

:  to run at low power and often disconnected usually so that power is not used for useful work

:  containing nothing

:  not occupied or inhabited

:  unfrequented

:  to make empty :  remove the contents of

:  deprive, divest

:  to discharge (itself) of contents

:  something (such as a bottle or can) that is empty

:  an expression of intention to inflict evil, injury, or damage

:  one that threatens

:  an indication of something impending

:  threaten