You may know it better as jalap, since jollop is principally a British spelling. It’s a liquid medicine of some sort, particularly cough syrup or a laxative.
“Listen,” said Granny, “If you give someone a bottle of red jollop for their wind it may work, right, but if you want it to work for sure then you let their mind make it work for them.”
Equal Rites, by Terry Pratchett, 1987.
The jollop pronunciation was known in English dialects for many decades before it began to be put into writing. A century ago, the English Dialect Dictionary found it in Lincolnshire and Lancashire and recorded that it then meant “a semi-fluid mess of anything; a big mess of food, a ‘dollop’.” That hints that it’s a variation on jalap, under the influence of dollop. The pronunciation is at least a hundred years older:
JALAP. The pronunciation of this word, as if written Jollop, which Mr. Sheridan has adopted, is, in my opinion, now confined to the illiterate and vulgar.
A Critical Pronouncing Dictionary, and Expositor of the English Language, by John Walker, 1791. The person he censures is not the playwright Richard Brinsley Sheridan, but his father, Thomas Sheridan, who published A General Dictionary of the English Language in two volumes in 1780. In it he did indeed suggest that way of saying the word (“dzhol-lup”). As the subtitle of his dictionary indicates (“One main object of which, is, to establish a plain and permanent standard of pronunciation”), Thomas Sheridan, an Irishman who was an actor as well as an elocutionist, intended by his work to teach the English how to speak. It is clear that John Walker was not amused (neither was Noah Webster, when Sheridan’s book became popular in North America at the end of the century).
Jollop has been recorded in American dictionaries as a slang term for a measure of strong liquor. The American Century Dictionary of 1895 said that it was an English provincial term for the cry of a turkey, which no British dictionary admits to knowing about. On the other hand, jollop was at one time a name for the wattles of the bird, probably from dewlap.
The older jalap arrived in English about 1675 via French from the Spanish purga de Jalapa, where the last word is an old name of the city in Mexico that’s now formally called Xalapa-Enríquez. It was a purgative obtained from the roots of a species of convolvulus.
Many readers wondered if there might be a link between the older jalap form of this word and either julep or jalopy. A julep, before it was that minty drink that I associate with Scarlett O’Hara, was a sweetened liquid medication, so in that sense there’s certainly a connection. However, there’s no doubt about the origin of julep (via French and Latin from Persian words meaning “rose water”) and the two words are etymologically unconnected. As to jalopy, the origin of this US slang term for a dilapidated old car is unknown, though one of the many stories that tries to explain it does unavailingly try to link it with Jalapa in Mexico.