Marx, in Theories of Surplus Value, quotes James Stuart writing about ‘profit upon alienation’.
When one sells a good one owns, one has alienated it from oneself. In the typical work relation under capitalism,
the managers of firms, that is, the representatives of capitalists, sell the product or services that workers
produce. Workers do not own the goods they produce, for they have previously sold their labor-power.
That is, they have agreed that their product is not owned by themselves, and neither is their labor.
But alienation means something more in Marx’s Paris Manuscripts.
The work of Ludwig Feuerbach is important background here. (By the way,
this post is based mostly on secondary and tertiary literature, I forget which. I have not read most of the Marx and Engels’ works cited