https://en.wikipedia.org/wiki/Das_Kapital

"Capital" in English, was a key text written by Marx, his whole notion of violent revolution by the proletariat is based on the premise of capital being concentrated unethically by certain segments of the society.

But you are right. The expression existed well before Marx. However, Marx certain came to frame the meaning of the term, which many have adopted.

The initial use of the term "capitalism" in its modern sense is attributed to Louis Blanc in 1850 ("What I call 'capitalism' that is to say the appropriation of capital by some to the exclusion of others") and Pierre-Joseph Proudhon in 1861 ("Economic and social regime in which capital, the source of income, does not generally belong to those who make it work through their labor"). Karl Marx frequently referred to the "capital" and to the "capitalist mode of production" in Das Kapital (1867). Marx did not use the form capitalism but instead used capital, capitalist and capitalist mode of production, which appear frequently.

Due to the word being coined by socialist critics of capitalism, economist and historian Robert Hessen stated that the term "capitalism" itself is a term of disparagement and a misnomer for economic individualism. Bernard Harcourt agrees with the statement that the term is a misnomer, adding that it misleadingly suggests that there is such as a thing as "capital" that inherently functions in certain ways and is governed by stable economic laws of its own.