Tolerably: adv.

• in an acceptable (but not outstanding) manner; "she plays tennis tolerably"

• acceptably, so-so

It means to an acceptable extent. The extent to which disgust or vileness are acceptable is a matter of personal opinion.

Firstly, an "old maid" is an old-fashioned - sometimes unkind - way of referring to an unmarried woman in a derogatory way. The neutral word would be "spinster" (also old-fashioned). A "maid" - without "old", would usually be a domestic servant. The assumption was often that "old maids" would be fairly strict, unattractive, unfashionable, often rather sharp or aggressive in character, or dull and old-fashioned - and sometimes rather masculine in behaviour. It was much more common in the 19th century and early 20th for two women who never married, sometimes sisters, (as is the case here), to live together. This sentence comes from the novel Huckleberry Finn, written in the 19th century by the American author, Mark Twain. In this context, the meaning of the adjective "tolerable" would be something like "acceptable" - either physically, or from the point of view of character - or both. At first, I didn't think the word referred to slim, as in that case we would have expected the writer to use the adverb, "tolerably". However, as "seand645" points out, "tolerable" would sometimes be used as an adverb, especially in the 18th century and continuing in dialect form into the 19th - in which case the word could well refer to "slim". This would also account for the absence of the comma after "tolerable", which we would expect to see after an adjective. If you want to see a picture of Miss Watson, have a look here: https://twain.lib.virginia.edu/huckfinn/frames3.html