What makes albert einstein come up with the word "gravity"?

Question

I mean, how does he know that the word gravity is gravity...

Answer 1

First, Albert Einstein did not come up with the word gravity. It was Isaac Newton who first gave 'gravity' the meaning we associate with it today, that is,  the force of attraction between two or more masses. As you can see from the quote below, Newton merely chose a word that described the force of attraction accurately; it is gravity that makes us feel 'heavy' on the surface of the earth.

"Our word gravity and its more precise derivative gravitation come from the Latin word gravitas, from gravis (heavy), which in turn comes from a still more ancient root word thought to have existed because of numerous cognates in related languages. For example, compare the Old English word grafan (grave), the Old Slavic pogreti (to bury), the Sanskrit guru (weighty, venerable), and Greek barus (heavy, grievous) among others. These words have common meanings of heaviness, importance, seriousness, dignity, grimness; the modern, physical sense of a field of attraction did not appear until Newton's time. Indeed, for Galileo, Newton, and scientists up to the beginning of the twentieth century, gravity was no more than an empty name for the phenomenon, a fact that they were well aware of."

Answer 2

I think the one you're looking for is Newton.  Newton is the one who came up with the universal law of gravitation.

You'd have to read Newton's 'Principia' to find out his thought process, but my guess is that, as Principia was probably written in Latin, he used the latin word for heavy ('gravis') and just tooled with it a bit.