A City Plum Is Not A Plum
by Christina Rossetti
A city plum is not a plum;
A dumb-bell is no bell, though dumb;
A party rat is not a rat;
A sailor's cat is not a cat;
A soldier's frog is not a frog;
A captain's log is not a log.
A captain's log, or log-book, is an official journal and record of the ship's progress; it was called such because you would take measurements with a block of wood at the end of a rope. A sailor's cat is an instrument of punishment, a whip, a cat-o'-nine-tails; it was also known as the captain's daughter. A soldier's frog, also known as a sword-frog, holds your sword or knife to your belt. A dumb-bell is not what you think: it is practice equipment for changeringing -- that is, the English practice of ringing church bells. Changeringing is hard work, so requires practice to develop your skills, but you don't generally practice on the bells themselves for obvious reasons. You use weights on ropes -- hence the word 'dumb', i.e., mute, and 'bell'. Our use of the term to apply to weight equipment seems to be a transferred use of this term.
What, then, is a city plum or a party rat? I don't know for sure; but this says a party rat is a politician who deserts his party and a city-plum is a rich person, which are plausible (but at the same time its claim that a soldier's frog is an ornamental fastening is quite implausible).