What is negative code? - Stack Overflow: "It means reducing lines of code, by removing redundancies or using more concise constructs.
See for example this famous anecdote from the original Apple Lisa developer team:
When the Lisa team was pushing to finalize their software in 1982, project managers started requiring programmers to submit weekly forms reporting on the number of lines of code they had written. Bill Atkinson thought that was silly. For the week in which he had rewritten QuickDraw’s region calculation routines to be six times faster and 2000 lines shorter, he put '-2000' on the form. After a few more weeks the managers stopped asking him to fill out the form, and he gladly complied."