The other day, my dad asked about the origin of humdinger. The OED says "origin unknown" but the entry has not been updated since 1976. Google's Ngrams seem to have some spurious early entries.
My dad used this word all of the time.
I refer to Online Etymology Dictionary. While its origin in a bit uncertain, there is a credible possibility described, as from hummer and dinger, both meaning really good:
humdinger
Is a humdinger a bodacious thing?
Definitely synonymous with "doozy." Â Both words used frequently by my dad.