Plum, in this case, does not refer to the fruit, but is rather an adjective meaning "good"... maybe. Plum may also be a reference to sugarplums, which was simply a sweet sort of candy, or to the raisins that were often used to sweeten the pudding, and were sometimes referred to as plums.

A plum pudding is a sponge pudding made with fruit and brandy and sweetened with sugar, although the details have varied greatly over the centuries. Early recipes used beer and beef; sometimes the sugar is mixed in, sometimes poured on top; and they were traditionally boiled, but the Victorians preferred them steamed. They have always been highly spiced.

"Boiled plum or Christmas pudding: Cut a pound of beef-suet extremely fine, to which add a pound of raisins well stoned, half a pound of currants, picked, cleaned, and dried, some nutmeg, two spoonsful of brandy, two ounces of candied lemon-peel, and one ounce of candied orange-peel

"Cucumberish
Etymology: cucumber +‎ -ish
Adjective: cucumberish (comparative more cucumberish, superlative most cucumberish).
(informal) Resembling or characteristic of cucumber."
Wikitionary

Cucumberish is a bit of Victorian slang that was first recorded 1822.¹ It was used exclusively to refer to people who were cool, in…

"Merry Andrew: MERRYMAN. The jack pudding, jester, or zany of a mountebank, usually dressed in a party-coloured coat.
--1811 Dictionary of the Vulgar Tongue, by Francis Grose.

A Merry Andrew is a somewhat mysterious type of clown. The Merry Andrew was a fixture of the Bartholomew Fair for centuries, presumably being the 17th century equivalent of a stand-up comedian.

"August 29. Met my wife in a