Laudator Temporis Acti

Letter from Madame d'Arblay (Fanny Burney) to her son Alex, a student at Cambridge University, from Joyce Hemlow et al., edd., The Journals and Letters of Fanny Burney (Madame d'Arblay) 1791-1840 (1972-1984), vol. 9, p. 76:

Have you Cocoa?
Have you got Shoes?
Do you take a Rhubarb pill from time to time?
an analeptic if you have any headache?
a black one if Nature is coy?

Nature's coyness is constipation, and rhubarb is well-known for its laxative properties.

The word rhubarb has an interesting history. See the Online Etymology Dictionary, s.v. rhubarb:

c.1390, from O.Fr. rubarbe, from M.L. rheubarbarum, from Gk. rha barbaron "foreign rhubarb," from rha "rhubarb" (associated with Rha, ancient Scythian name of the River Volga) + barbaron, neut. of barbaros "foreign." Grown in China and Tibet, it was imported into ancient Europe by way of Russia. Spelling altered in M.L. by association with rheum. European native species so called from 1650.

Ammianus Marcellinus mentions the plant (22.8.28):

Nearby this is the river Ra, on whose banks grows a certain invigorating root of the same name, which is beneficial for various curative applications.

huic Ra vicinus est amnis, in cuius superciliis quaedam vegetabilis eiusdem nominis gignitur radix, proficiens ad usus multiplices medelarum.

The plant's genus is Rheum, and the native European species is Rheum rhaponticum. According to the New American Heritage Dictionary of the English Language, College Edition (1979), "the dried, bitter-tasting rhizome and roots of R. palmatum or R. officinale, of Central Asia" is "used as a laxative." Calvert Watkins' Indo-European Roots, an appendix to this dictionary, says s.v. ers-²:

To be wet. 1. Variant form *ros- in Latin rōs, dew: ROSEMARY. 2. Suffixed variant form *ros-ā in Avestan Ra(n)hā, name of a mystical river: RHUBARB. 3. Extended root ersen-, male (< "that sprinkles or ejects semen"), in Old Persian arshan-, man: XERXES. [Pok. 2. ere-s- 336.]

Pok. is a reference to J. Pokorny's Indogermanisches Etymologisches Wörterbuch.