According to arch-hacker James Gosling, to "...modify a program with no idea whatsoever how it works." (See whacker.) It is actually possible to do this in nontrivial circumstances if the change is small and well-defined and you are very good at glarking things from context. As a trivial example, it is relatively easy to change all "stderr" writes to "stdout" writes in a piece of C filter code which remains otherwise mysterious.
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Nearby terms: WFWG « WG « WGL « whack » whacker » whales » whalesong