Yes, Beth is right. But it’s also standard English. See Webster’s Second, plum, 7: "Something likened to a plum in desirableness; a good or choice thing of its kind, as among appointments, positions, parts of a book, etc.” Or W3: 4b: "something excellent or
superior of its kind (as a choice passage in a book or an unusually good position)”; 4c: “something desirable received or available as a recompense for service.”
Brian Boyd
On 17/04/2014, at 12:51 pm, Don Johnson <chtodel@COX.NET> wrote:
Susan’s answer re Jansy’s query is certainly on target. I would add that “plum” as in “plum good” or “plum smart” was quite
common in my semi-ruralIndiana childhood---possibly echoing its use in the old English childrens’ rhyme. Don Johnson
In response to Jansy's inquiry, I assume that Nabokov has in mind the English nursery rhyme
"Little Jack Horner," in which the title character pulls out a hidden plum from a pie and then congratulates himself.