Whiteknighting

"Whiteknighting" is a term used largely by anti-feminists to describe the situation when a percieved hetero man comes to a woman's defense in a heckling situation. See any internet argument debate mentioning Anita Sarkeesian or Rebecca Watson for examples.

Use of the term "white knight" in other circles is not covered in this article.

Uses
The term is used in a number of contexts, but its primary aim is intended to attack the man as unnecessarily and dishonestly covering for women. It refers to the trope of a "white knight" and supposes that the woman is perfectly capable of defending herself; that, by "coming to her rescue", the knight is undermining her, which is counterproductive to the woman gaining any sort of respect. In some cases, the man intends to curry favor with the woman in order to seduce her — regardless of whether this is actually true. When this kind of behavior is for real, it's usually perceived as creepy, manipulative-bordering-on-stalking Nice Guy™ behavior and is almost always unwelcome, making the perceived sting of the accusation worse. However, even in the unlikely scenario that seducing the woman really is the man's intention, it still would not automatically make him wrong.

In many cases, however, this is often misused as a thought-terminating cliché to attack any man expressing concerns about sexism. It's a byproduct of the dickwad theory as applied to sexism, used as an attempt to shame defenders of a woman into backing off. This allows the hecklers to harass her by implying that the defenders must have an ulterior motive. When successful, it can be considered a form of heckler's veto. Either that, or it is just simply used as a method of attacking the person making an argument rather than their actual arguments.

When the woman being defended does not conform to mainstream standards of beauty, so that it seems incomprehensible why someone would want to seduce her, an alternative explanation put forth is that the defender lacks experience with women and therefore naively idealizes them. Both kinds of ad hominem arguments focus on the defender's alleged romantic failings.

Accusations of being a white knight are very Godwin's Law-esque; if any guy speaks out against sexism directed towards women on the internet, it is inevitable that somebody will accuse him of being one.

Given the faceless anonymous nature of the internet, chances are also good that anti-feminists have hurled this accusation at women (or people not attracted to women in general) without knowing so. It can get awkward for the accuser if the opponent tells them, though the accuser will likely later use the typical "feminazi" or "western woman" insult without reevaluating the merit of the original "white knight" insult.