noun
complete mastery; at one's beck and call
Idiomatic expression meaning to have something or someone completely under one's control, like a medicine in one's own chest, ready to be used at will. Rare and literary; not used in everyday conversation.
彼はその技術を薬籠中の物としている。
He has complete mastery of that technique.
あの政治家は官僚を薬籠中の物にしていると言われる。
That politician is said to have the bureaucrats at his beck and call.
Similar meaning of having something under one's control, but 掌中に収める is more common and less idiomatic.
From classical Chinese; literally 'an item in one's medicine chest', implying something always at hand and under one's control.