In the same way, too, man in general, abstract man, in a sense is only an abstract generality, but this abstract generality begins to exist concretely in individuals detached from any particular tie which capitalist society creates, in the modern proletariat in which the fully developed individual of future communist society prefigured. This objective flowering of the general in the form of particular existence is the corner-stone of the dialectical materialist conception of the development of the concept and movement in science. Not, of course, that we must confuse the real concrete with its mental representation in the form of the concrete-in-thought, a confusion characteristic of speculative idealism as well as pre-critical empiricism.
It's a purely synthetic construction; "_____-in-thought" by itself does not have a general meaning in the English language when used that way. The author is referring to a specific idea put forth or constructed in some other passage.
(In philosophy sometimes philosophers will simply say things like this when they are defining them and leave the reader to detect that use - I think this is extremely sloppy! But I can tell you for sure there is a whole theory of the 'concrete-in-thought' in the so-called 'dialectic materialism'. If it makes any real meaning in your head or not after you read the primary sources is for you to discover....)