also: ごまのはえ
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.
At Edo-period inns, people were warned to be careful of thieves posing as fellow travelers.