The story
- Proto-Indo-European*Hrew-?
- Proto-Indo-European*Hrewp-
- Proto-Germanic*raubōną
- Frankish*raubōn
- Late Latinraubāre
- Old Frenchrober
- Old Frenchrobeor
- Middle Englishrobbour
- English todayrobber
Inherited from Middle English robbour, robbere, either directly taken from or from a calque of Old French robeor. Equivalent to rob + -er. Compare reaver (“robber, plunderer”), a native English word derived from Proto-Germanic *raubārijaz that is ultimately of more or less the same composition as robber. And compare rover (“a pirate”), another word of the same composition.
Etymology from Wiktionary, CC BY-SA 4.0.