i think a non-japanese person with a japanese username, an anime girl as their avatar, near exclusively plays japanese games (mostly anime or nintendo at best, hates on western games and especially the cinematic/shooter style ones) and tends to use japanese words even though they know 99% of people they are talking to dont understand the word
they usually "claim" to be a fan of japanese culture too, but the culture they are aware of is what they see in harem anime and stereotypes of what a salaryman is. You never see them being huge fans of the more standard live action tv dramas and or music that isnt j-pop (specifically you wont see them being fans of a male japanese rock band or something like that). Its just a highly fetishized view of what japan is like and thats what i think best describes a weeb. Not simply a literal japanophile, but an ignorant steretyping of "what japan, my favorite country that i have idolized because of anime, is like".
its like someone in japan if they were interested in colonial america and researching that. its fine and nobody is gonna see that as weird. But the japanese guy who views all americans as what the big bang theory is like, wants to dress in "authentic american clothing" (a native american outfit) and watches nickelodeon all day because "its so much better than japanese tv" and so on? he'd be like a weeb for america, as its just that "ignorant stereotypical fanboy" view of a country that people actually in the country find dumb/annoying
(i'm more bothered by these type of people as they tend to match up with the hardcore anime otaku crowd, which has negatively influenced at least anime and gaming to be worse than it could be. i dont mind if that makes me bitter)