From "oppai" おっぱい, the Japanese have derived other terms using "pai" to describe ways that boobs get shiok shiok by certain actions:
Example one:
Paisura パイスラ （Rubbing of the boob valley by a suspended slash or seat belt) pai パイfrom "oppai" おっぱい (breasts) + sura スラ from surasuスラス(slash)
Take a look at how enjoyable is the big boob girl having a seat belt to go up and down her boob valley.
kimochi kimochi