"Bogo" is derived from Bogo (also called tsuchinoe, uma), the zodiac sign of that year when the chokusho was given, and "mitchoku" or a secret Imperial command means that the document was given without going through the official procedure which required to obtain approbation from Kanpaku (chief adviser to the Emperor), Hisatada KUJO.
「戊午」は下賜された安政5年の干支が戊午（つちのえ・うま）であったことに由来し、「密勅」とは正式な手続（関白九条尚忠の裁可）を経ないままの下賜であったことによる。