>>104514638This phrase is a subtle joke.
The guard says "no lollygagging", an English word referring to aimless idling or loitering, especially around a place of business. The way the phrase is pronounced is similar to the combination of the words "loli", a reference to young women of questionable ages, and the word "gagging", a reflexive response to undesirable objects entering one's throat. Thus, as Muyu may be considered a loli by many, she produces a gagging noise to act out the similarly pronounced phrase "loli gagging" as a euphemism for intense fellatio performed at an uncomfortable depth in response to the guard's statement forbidding the former action to question if the latter is allowed. 