Plump comes from a dutch word meaning blunt, not sharp, rounded. It now means full with fruits or overweight with people. It has become something of an insult, equating with being fat. You can plump up cushions or pillows to make them fuller and more rounded.
Plum, the fruit, may be plump, but the origin of their name is not dutch. It seems that the ancient Greek proumnon (prune) evolved into Plum by the time it reached here as Old English.
A plumb may be shaped like a plum, but the word derives from the latin for lead; plumbum. A plumb is a lead weight attached to a line used in building to measure if a vertical is true. It is also used in sailing to measure the depth of water beneath a vessel.
This Is Just To Say: by William Carlos Williams
I have eaten
that were in
you were probably
they were delicious
and so cold