A bit of Japanese onomatopoeia.
"Pompoko" is the sound that tanuki (Asian racoon-dogs) traditionally make when beating on their distended scrotums (scrota?) in drum-like fashion. Thus the refrain of a very famous Japanese children's song about tanuki, "Shojoji no Tanuki Bayashi," is "Pom Poko Pom no Pom."
Sometimes the Japanese clean up the story for children and say that the tanuki are beating on their "tummies," but don't let that fool you - "pompoko" is definitely the dulcet tone of a well-struck scrotum.