Instant blindness, gritty panic, the world condensed into stinging darkness: mud in the eye. Children are quite familiar with this weapon, the sneaky trick that ends playground battles, leaving everyone weeping and none winning.
Biblical healers employed it otherwise. Mixing saliva with dirt, Jesus used mud to blind eyes and eyesight returned; the most improbable treatment, earth and saliva became miracles. Sometimes what seems to be an assault is actually preparation for seeing precisely.
Boxers metaphorically have dirt in their eye, cheap shots, below-the-belt strategies, battles that violate rules for survival. That was mud in the eye, they claim about betrayals, about buddies that smilingly honed knives behind their back. That expression implies insult, disrespect, premeditated humiliation. But real mud in real eyes is worse, you can't blink it away, can't wash it out easily.
Hoping they are honest and won't take advantage of your brief vulnerability, you are vulnerable and stumbling, reliant on whoever's nearby to point you to water.

