@skciD
Okay, so. Out of curiosity I did some digging, and I found what モリマン (Moriman) is.
Basically, it's when there is a protrusion around a female's crotch. Referred to as a Moriagatta Manko (盛り上がったマンコ) or shortened to Moriman (盛りマン), it is translated as "Vagina/Pussy Bulge". Although, it is technically referring to the outer part of the vulva called the Labia Majora; which is an external part of the vagina.
You can read the whole article here: https://dic.pixiv.net/a/%E3%83%A2%E3%83%AA%E3%83%9E%E3%83%B3
Anyways, just follow the link and look for some art to see what it is referring to.
PS: Most search results I found referenced Holstein Morio and Taneuma Man, a 1995 Female comedy duo called Moriman (モリマン) who are still semi-active today and were relatively famous in their prime.
Going off that lead I would've originally translated the line on ch4 p2 to be "It's just a prank", but it's a good thing (?) that I did some extra research. 😅