Matt, To answer you question, as far as I can tell the expression has two meanings. But don’t bet any money on these because I was only able to make inferences from usage after doing a Google search – wasn’t able to find the expression in any references.
As far as I can tell this expression had its moment in the sun, and perhaps its invention, in the 1989 Batman movie (also song on CD in the early 1990s). The Joker after shooting the guy who was messing with his romantic interest says: “Never rub another man's rhubarb. Why is it every time I come for you [Vicki], somebody always gets in the way?"
So one meaning, and the Batman meaning, is don’t rub, ‘don’t touch / mess with another guy’s girl/women.’
The second meaning, which seemed to be in the majority in the Google hits appeared to be the one you suggested, ‘don’t aggravate/irritate someone.’
Eric Partridge’s ‘A Dictionary of Slang’ gives one of the definitions of ‘rhubarb’ as ‘Genitals, male or, occasionally, female.’ ‘Cassell’s Dictionary of Slang’ says: [late 19th century and still in use] The genitals of either sex and thus the coarse query “How’s your rhubarb, Missus?” or “How’s your rhubarb coming up, Bill?
So it seems that in the Batman expression the ‘rhubarb’ is a stand-in for the girl/women’s private area, which in turn is a stand-in for the girl/women – wow a metonymic synecdoche! In the ‘aggravation’ case things are more straightforward – we have a direct assault on a stand-in (<:) that most guys don’t like receiving tactile intrusions on from other guys. And you thought that rhubarb was just about pies and squabbles!
Ken G – November 4, 2003
Reply from Ken Greenwald (Fort Collins, CO - U.S.A.)