See http://www.mingw.org .
Gloss mingwin as "Minimalist GNU for Windows" [L1 ]. Is "mingw32" an exact synonym? In any case, one often sees the latter, also. Some idiolects seem to favor "minimal" for "minimalist", also.
Most of the excitement linking mingwin and Tcl has to do with critcl and its potential, first to maintain portable extensions far more conveniently, and then to ... In this scheme, mingwin provides cross-generation and cross-linking (among other) capabilities.