to embezzle; to misappropriate; to peculate
to accustom oneself to using; to use for a long time
彼は会社の金を使い込んだ。
He embezzled the company's money.
職人の道具は使い込んで手に馴染む。
A craftsman's tools become familiar to the hand through thorough use.
新しい革のバッグは、使い込むほどに風合いが増す。
A new leather bag gains more character the more you use it.