One way to hold commissary: somebody higher up hands a job to somebody else and says, from here on, this is yours to run. The person holding that job is a commissary. The authority is borrowed and it has edges — it works only inside the job he was handed. In an army, the job that got handed over most often was food: buy it, keep it, give it out. From there the name slid off the man and onto the counter he stood behind.
From the word’s history
The supplied source gives Late Latin commissarius, from commissus, the past participle of committo, to commit or entrust. So the word names an entrusted person, which is exactly the top image here. The source also notes it is a doublet of commissar.
來源給的字源是晚期拉丁文 commissarius，來自 commissus，也就是 committō（委託、交付）的過去分詞。所以這個字字面上就是「被交付任務的人」，跟上面那個畫面剛好吻合。來源另外指出，它和 commissar 是同源的雙生字。
Wiktionary, "commissary"
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