Rhubarb : noun La rhurbarb is a plant with broad leaves borne by large petioles that we eat cooked.
Pass me the rhubarb, I'll pass you the senna (*): let's make mutual concessions. Refers to an elevator return, a service exchanged for another.
“The old expression is 'pass me the case, and I'll pass you the senna.' Two purgative plants. Rhubarb then replaced the case.
Thus, Edmond Rostand, in "Cyrano de Bergerac" (1897) replaces one of these two purgative plants by making Cyrano say "And donor of senna by desire for rhubarb"
(*) Senna: Shrub (legumes) producing pods from which a laxative drug is extracted; this drug.
Polish rhubarb: masturbate.