also: ごまのはえ
noun
thief posing as a fellow traveler
Historical term for a thief who pretends to be a travel companion in order to steal. The kanji 護摩の灰 literally means 'ashes of a goma fire ritual', while 胡麻の蝿 means 'sesame fly'; both are ateji-like spellings for the reading.
江戸時代の旅籠では、護摩の灰に気をつけるよう言われていた。
At Edo-period inns, people were warned to be careful of thieves posing as fellow travelers.
The origin is uncertain. The spelling 護摩の灰 (goma no hai, 'ashes of a goma fire ritual') is likely ateji, while 胡麻の蝿 (goma no hae, 'sesame fly') may be a phonetic rendering. The term is historical and rarely used in modern Japanese.