to have an idea, some plans, etc. that you keep ready to use if it becomes necessary
HAVE A TRICK, SOME MORE TRICKS, ETC. UP YOUR SLEEVE
Meaning of HAVE A TRICK, SOME MORE TRICKS, ETC. UP YOUR SLEEVE in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005