In Bakemonogatari, Mayoi says a tongue-twister that goes
("namamumi, namamome, namamamamo") then gets embarrassed because they sound like another word. What's the reference they're making?
Caution: the answer might be NSFW since it contains vulgar words.
Here is the full dialogue by Araragi and Mayoi:
Mayoi: So, say "namamumi namamome namamamamo" three times
Araragi: You can't even say it properly1
Mayoi: Namamome is lewd!
Araragi: It's because you who says it
Mayoi: Namamamamo is lewd!
Araragi: I don't understand what's lewd with that, though...
(translated by myself, if there's an official translation of this dialogue, feel free to correct it)
Namamumi doesn't exist in Japanese. The best Kanji representation is 生【なま】無味【むみ】, which means "raw tasteless".
Namamome can be represented by 生【なま】揉め【もめ】, which means "bare (breast) grope".
Namamamamo is... well, that word also doesn't exist in Japanese, and also meaningless. Even Araragi doesn't understand why Mayoi says that it's lewd.
1The original tongue twister is 生【なま】麦【むぎ】生【なま】米【ごめ】生【なま】卵【たまご】 (Namamugi namagome namatamago), which means "raw wheat, raw rice, raw egg". The one that Mayoi says is when all the consonants are changed to "m" (except the "n" in "nama"). This is to simulate the pronunciation by very young child (e.g. toddler) when their ability to speak is not well-developed yet.