PU

[Barnhart]

But, these academic/sports contexts among later teenagers and early 20
somethings is a holdover from the pre-teen expression.  I don't
remember seeing it written PU.  I do think that I probably stopped
using it about the time I shuffled from sixth to seventh grade (early
1950's)--about or shortly after the same time I stopped using the term
dog-do.

I'm suspicious of the derivation coming from putrid.  That's a pretty
adult word for an elementary school kid.

Regards,
David K. Barnhart