We use this expression regularly, but why is it that something like this is said when we want to express the suspicion we feel towards something or someone ?
In general , the irritating behavior of this insect is pointed out as the origin of this idiom , when it gets heavy and flutters around us, causing a buzz that gets to upset us and encourages us to hunt it down .
However, another hypothesis affirms that when they mentioned the fly they were referring to the match or key with which the arquebusiers and musketeers used to fire their weapons.
Between shots, the shooters held the lit fuse behind their ears , as if it were a pencil or a cigarette, while refilling the powder. And it was in that moment of defenselessness when the soldier was on alert for the slightest danger, since he had "the fly behind his ear."
