A plummet, as you may know, is a little bit of lead (hence the name: from plumb, from Latin plumbum ‘lead’, plus diminutive –et) used to weight a line for sounding depths or determining vertical. It is also a word for a stick of lead for writing with. We have had the noun since the 1300s. The verb plummet first (in the early 1600s) meant to use a plummet to sound the depth of water; more recently (from the mid-1800s) it has meant to fall precipitously – like a plummet being dropped, I guess. It has nothing to do with plums… except when they fall from the tree, of course.
Here is a poem. I hope it goes down well.
If you would lead, then bring your lead
and take your soundings from it—
you need not heed what’s in your head;
just stop and drop your plummet.
If to the object you object,
don’t seek to overcome it;
your sense is suspect, I suspect—
don’t guess the depth, just plumb it.
When you have read what you must read
from bow or bridge or summit,
then learn what’s plead and once more plead,
and look before you plummet.