bone up on

英語の勉強をしている私としては覚えておきたい慣用句がPLAYBACKに出て来ました。
He was a rapid eater even when he talked too much. He pushed his plate away from him, drank some of his coffee and got a toothpick out of his vest.
"This is a rich town, friend," he said slowly. "I've studied it. I've boned up on it. I've talked to guys about it. They tell me it's one of the few spots left in our fair green country where the dough ain't quite enough. In Esmeralda you got to belong, or you're nothing.
"bone up on" を辞書で調べます。
・Cambridge English Dictionary: informal Study (a subject) intensively, often in preparation for something: she boned up on languages she had learned long ago and went back to New Guinea ・Dave's ESL Cafe: (inseparable): review / study thoroughly for a short time.: "If you're going to travel to Peru, you'd better bone up on your Spanish."
この慣用句の由来は： ・The Phrase Finder: Origin: There are two chief theories as to the origin of this phrase. One is that it derives from the practise of using bones to polish leather. So, to 'bone up' on a subject was to polish or refine one's knowledge. The second theory relates to the Victorian bookseller Henry George Bohn (1796-1884). He produced a large catalogue of books, including many study texts.
この後の説明は長くなるので省略しますが、"bones to polish leather" 説の方が信憑性が高いとしていました。
上の語源についての記述を読んでいたら思い出しました、この表現 "bone up" は昨年（1/8/2014）既に取り上げていました。しかもコツコツと勉強すると覚え方まで考え付いたのに忘れているとは、情けない。