一切經音義 9.226

9.226

級其（羇立反禮記級階次也左傳加勞賜賜一級又云斬首二十三級案師旅斬首一人賜爵一級因名賊首為級也）。

級其 (fanqie: 羇 + 立. The Liji says: '級 means steps or ranks'. The Zuozhuan says: 'For merit, he was rewarded with one rank (級)', and also: 'Severing twenty-three heads (級)'. According to this, when an army beheaded one enemy, they were rewarded with one rank of nobility; hence an enemy's severed head came to be called a 級).