To do something "with aplomb" is to it with poise, self confidence and assurance. On the other hand, if something is "leaden" it is dull, heavy, slow. Like lead. Oddly, these words come from the same source. Not quite brothers, these words are first cousins.
They both branch off the Latin plumbum which was the word for the metal lead. As pipes were often made of this the men who worked with them were, and still are, called plumbers. Its a bit of useless medical trivia but lead poisoning is called plumbism.
Aplomb comes from the French a' plomb which meant "upright, straight, balanced". It was a reference to the unwavering straight line you could define with a plumb bob, which is to say the lead weight at the end of a string commonly used by stone masons.
I once dug a Roman plumb bob. It was one of my early years at Vindolanda so I don't have a decent photo. Like all things lead it came out of the ground with a light coat of lead oxide that looks a bit like a dusting of flour. And of course as soon as you pick it up its weight tells you what you have. Here's a photo of a Roman plumb bob that was dug at Catterick, about a hundred miles further south. A modern example is helpfully along side.
And here's a brief history of the plumb bob courtesy of Wonkee Donkee Tools!