A POSIX 1003.2 (draft 11.3) compliant interpreter for the AWK (Aho, Weinberger, Kernighan) programming language.
The "M" in "mawk" probably stems from the first name of the author of mawk, Mike Brennan. In the manual page for mawk it is mentioned that "Implementors of the AWK language have show a consistent lack of imagination when naming their programs".