Kinda late at explaining this but for those still wondering, in Japanese, the joke in the first chapter goes like this:
They made a sign with "ち〇こ" (Chi-?-ko) on it and asked around what character the "〇" was. Those who thought dirty guessed "ん" (n), so you'd get "ちんこ" (Chinko) or "penis". The actual answer was "ょ" (yo), so you'd get "ちょこ" (Choko) or "chocolate". And then, there was Shijou who guessed "ちまなこ" (Chimanako) or "bloodshot eyes/in a frenzy".
Props to the translator for managing to translate that in English.