shy bowel

(名詞) 害羞的腸子:在公共廁所無法排便 (想拉但拉不出來) 的狀況,又稱公廁恐懼症 (public restroom phobia)。

引句中的 number two (二號) 為大便的委婉語 -- 小便的委婉語叫做 number one (一號);它們常見的搭配有 to do number two/one 和 to make number two/one (上大號/小號)。再者,引句中的 behind 當名詞用,為口語的「屁股」。

Parcopresis is the phobia assigned to those who absolutely cannot do number two in a public restroom. This phobia is also termed as “shy bowel” because the sufferer, your date, must have a certain level of privacy before their mind will let their behind “let loose.”
—Michele Gwynn, "Public restroom phobia cripples dating,", August 20, 2009