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The kilogram is mass, not weight. Weight is force. Force is measured in Newtons.

4 cups, you have had a quart of coffee, sir ;)

First they defined the meter and then one litre was defined as one dm^3.

That’s an interesting question that I’d never thought about before.

I asked chatGPT, which predictably bullshitted me and said they’d decided grams made more sense than kilograms for scientific lab work.

But then I searched and found this from the user tomalator on Reddit:

“When the French were developing the metric system, they suggested the unit be called a grave (pronounced grav) being the mass of 1L of water (1000 cm3)

The French at this time being in the middle of a revolution against the rich notice that it sounded a lot like the word Graf, being a word for Duke or Earl, and they wanted to avoid affiliating with the nobility, so they changed the measurement to be the mass of 1mL of water (1 cm3) and called it the gramme

They then noticed that it was inconvenient to use a mass unit so small, so they changed back to the 1L of water definition, but kept the name gramme for the base, and threw out the word grave in favor of the kilogramme.

And that's why the kilogram is the base SI unit and not the gram. I had the exact same question when I learned the SI unites.”

A gram is not the base unit, it started with one meter (hence, metric).

Kilo means thousand in Latin, so 1000 meters became one kilometer (aka, one thousand meters), and when they need smaller units, they took to Latin again, simply because the language was en vogue for science:

Deca (ten): Decimeter (dm, nowadays hardly used, but it exists) = 0.1m

Centum (hundred): Centimeter (cm) = 0.01m

Mille (thousand): milimiter (mm) 0.001m

Weights were then adopted from the dimensions based on practicality, i.e. one liter was a common enough volume that people could use it in a household, and it's defined by 1dm height x 1dm length x 1dm width. Or 10cm10cm10cm (same thing, but the base notation was units of one).