What does Boxing Day have to do with boxing?
late 14c., from Old French rubarbe, from Medieval Latin rheubarbarum, from Greek rha barbaron "foreign rhubarb," from rha "rhubarb," perhaps ultimately from a source akin to Persian rewend "rhubarb" (associated in Greek with Rha, ancient Scythian name of the River Volga) + barbaron, neuter of barbaros "foreign" (see barbarian). Grown in China and Tibet, it was imported into ancient Europe by way of Russia.
Spelling altered in Medieval Latin by association with rheum. European native species so called from 1640s. Baseball slang meaning "loud squabble on the field" is from 1938, of unknown origin, said to have been first used by broadcaster Garry Schumacher. Perhaps connected with use of rhubarb as a word repeated by stage actors to give the impression of hubbub or conversation (attested from 1934).
A loud quarrel or squabble; a controversy of riotous potential, esp among baseball players on the field: beanball throwing, rhubarbs, and umpire baiting
[1938+ Baseball; origin unknown and richly speculated on; said to have been first used in a broadcast by Garry Schumacher]
A low-level aerial strafing mission (WWII Air Forces)verb
: flying for rhubarbing
[an arbitrary code name]