As soon as I saw the reports about the mobile PaPaPa vans roaming the streets of Chengdu (see "PaPaPa" [2/15/17]), I immediately thought of a similar expression with a similar meaning that I heard forty years ago. On that occasion, someone described to me the actions of a man who was trying (unsuccessfully) to get an erection as "PiaPiaPia". Since that was the first time I had heard that expression, I didn't know for sure what it meant, but I could pretty well guess.
The person who used that expression spoke the purest, clearest Pekingese I have ever heard, but what struck me particularly about "PiaPiaPia" is that the sounds of which it is composed are not included in the standard inventory of possible Modern Standard Mandarin (MSM) syllables.
Even though theoretically "pia" is not a permissible MSM syllable, still it is widely spoken. The question arises of how to write this word in the spoken language in characters. The usual way is simply to use Pinyin: pia. Some people say it is 1st tone, some say it is 4th tone. It is onomatopoeia for the sound of slapping. It is often used with reference to slapping someone in the face and can also be used to describe someone making a pratfall. It usually occurs in threes: piapiapia!
The online encyclopedias baike has an article about the origin of this expression (from the Northeast / Manchuria) and the consternation people feel over how to write it. Some people resort to the Sinographic makeshift of pā 啪, and there have been other attempts to write "pia" with characters, but they are all unsatisfying to those for whom "piapia" or "piapiapia" is a kǒutóuchán 口头禅 ("stock / pet phrase; mantra; favorite expression"). Most people simply use Pinyin and write it as "pia".
The Baidu encyclopedia article on "piapia" is brief and confusing.
Here are some other buzzwords that are outside of the standard syllabic inventory for MSM and do not have a fixed orthography in characters:
“biaji-biaji” — used in connection with noisy eating, especially with regard to people smacking their lips; akin to “om nom nom” in English (where did THAT come from?); sometimes written as bāji bāji 吧唧吧唧 ("yum[my] yum[my]")
"QQ" — ubiquitously written just like that, but theoretically might be pinyinized as kiūkiū (that's the way it is typically pronounced); for the various meanings and derivation of QQ, see "A New Morpheme in Mandarin" (4/26/11)
"muà" — a kissing sound, borrowed from English "mwah"; I don't know of any attempts to write this in Chinese characters.
Here are some earlier posts on words of this type:
"Kiss kiss / BER: Chinese photoshop victim" (7/22/14)
"More on "duang"" (3/19/15)
"Creeping Romanization in Chinese" (8/30/12)
"The Awful Chinese Writing System" (Geoffrey Pullum, The Chronicle of Higher Education, 1/20/16)
When Mandarin speakers resort to Pinyin to write sounds and morphemes that do not exist in the standard syllable inventory for MSM, that is one important factor in the emerging digraphia to which we have often referred here on Language Log and elsewhere.
[Thanks to David Moser, Jonathan Smith, Brendan O'Kane, Fangyi Cheng, Yixue Yang, and Jinyi Cai]