According to WNT this derivative of uncertain origin could be identical to Pijpzak [Which is a Belgian (!) synonym of Doedelzak; NB¹: ➺ Dutch Pijpsack and Pypsack; ws]; This is, however, a hypothesis that only can be sustained if the now obsolete Dutch vernacular expression "in de piepzak zitten" [to have the wind up / to be tense or nervous (Am.: to be in a (blue) funk)] would be related to the fact that the bag of a bagpipe is being squeezed when playing, which would cause anxiety to anyone inside it; NB²: Compare Zakpiep.