Within leftist circles, the term originates from the more formal use of "Social Chauvinists" who were the so-called socialists who sided with their own individual nations when WW1 started and cheered on the war, instead of siding with international workers and opposing the war across the board. It's basically the worst insult that could be levied by a leftist outside of fascist, it's stating that one has completely abandoned internationalist principles and solidarity for selfish warmongering short-term gains.

So a "western chauvinist" is a westerner who holds heavy pro-west biases and anti-global south biases. They don't even have to be conscious or purposeful. Simple holding the generic American worldview is sufficient to be a chauvinist. Just passively absorbing the dominant racist, imperialist, colonialist ideology of the West will make one a chauvinist if one doesn't introspect and cleanse oneself of it.

It's been show through history that the leftist groups that tend towards chauvinism are almost always the most "Liberal" and reformist of the lot. The "Social Democrats" like Bernie Sanders always side with international capital and zionism because they are traitors. It's generally the more radical communists who maintain their principles of internationalism. This is a major source of conflict between the reformist and revolutionary lefts, whether to maintain ones empire or not.

In poor and oppressed and colonized nations this divide is not as prominent. Everyone who supports decolonization or is on the left is easily united under a single banner of liberation. In imperialist western rich countries though, the left splits apart into the true comrades and the sell outs who really believe in their own superiority.

You see this chauvinism among the left manifesting in a lot of ways. The most common is that we (as in white rich nations) are the only people who can do revolution peacefully and correctly - that the “others” did socialism wrong and we know better than them. Another common strain is the nihilistic trend of “well everyone is evil and imperialist so who even cares”. This lazy both-sidesism projects what the western left knows is bad about their own society onto other societies they know nothing about, to create false equivalence and make it ok to just sit around doing nothing. Another kind of chauvinism is just unironically believing American propaganda about the “authoritarians” of the East.

You were doing a mix of 2 and 3