So in an argument, if someone “gerrymanders” their reasoning, they might selectively define terms, group evidence, or draw distinctions in a very convenient way so everything appears to support their position.

For example:

> “You’re basically gerrymandering the definition of ‘success’ so that every outcome counts as evidence that you were right.”

Or:

> “He gerrymandered the categories until the data fit his conclusion.”

The key idea is **not merely being biased**. It implies *artificially arranging the boundaries of the discussion* to engineer a favorable result.

Depending on exactly what the person is doing, related terms are **cherry-picking** (selecting only favorable evidence), **moving the goalposts** (changing the standard when challenged), **special pleading** (creating convenient exceptions), and **motivated reasoning** (reasoning toward a conclusion you already want). “Gerrymandering” is especially apt when the manipulation involves **how things are divided, classified, defined, or framed**.

(http://www.autoadmit.com/thread.php?thread_id=5892624&forum_id=2most#50064949)