*drūpaną

Probably a secondary strong verb, back-formed from the iterative *druppōną (‘to drop, fall; to drip, seep’) itself from a pre-Germanic nasal formation *dʰr̥bʰ-néh₂-ti ~ *dʰr̥bʰ-n̥h₂-énti from Proto-Indo-European *dʰrebʰ- (‘to drop’). The root-final *p reflects abstraction from the Kluge-law geminate *pp in the iterative. The long vowel of *drūpaną is an analogical Strong class 2b remodeling of the zero-grade base *drup-, beside Class 2a *dreupaną (‘to droop; drip’). Compare *drupô (‘a drop’).