something always at hand; someone at one's beck and call; complete mastery over something
A yojijukugo (four-character idiom) meaning something or someone one can freely use or control at any time, like having one's own medicine cabinet. Often used in the phrase 自家薬籠中の物 (じかやくろうちゅうのもの).
He has complete mastery over knowledge in every field.
彼女は優秀な部下を自家薬籠中の物にしている。
She has excellent subordinates at her beck and call.
From the idea of having one's own medicine cabinet (自家薬籠) from which one can freely take remedies. The exact historical origin is uncertain, but it is a classical Chinese-derived idiom.