WORD. Seriously, I get the irritation when it's some obscure phrase that isn't explained, but if it's just a word that's either generally known in the fandom or otherwise fairly easy to find the meaning for (if it isn't mentioned), then what's the big deal? Maybe it's just because I used to do it myself back in the InuYasha fandom ("kazaana" sounds so much cooler than "wind tunnel," honestly people), but I don't see what the problem is if it's appropriate.
Though I do stress the "when it's appropriate" part. My biggest pet peeve is people using Japanese terms in dialogue for fandoms that aren't set in Japan and where the characters don't actually speak Japanese. Case in point: If the series hadn't been written for a Japanese-speaking audience, then Alphonse Elric wouldn't have been calling his brother "Niisan" at all. Because, shock upon shocks, they don't actually speak Japanese in Amestris. It's all but outright stated that they don't, but you'd think more people would figure it out when Ed ended up in England and could, y'know, understand what everyone was saying.
#40 TL;DR
Though I do stress the "when it's appropriate" part. My biggest pet peeve is people using Japanese terms in dialogue for fandoms that aren't set in Japan and where the characters don't actually speak Japanese. Case in point: If the series hadn't been written for a Japanese-speaking audience, then Alphonse Elric wouldn't have been calling his brother "Niisan" at all. Because, shock upon shocks, they don't actually speak Japanese in Amestris. It's all but outright stated that they don't, but you'd think more people would figure it out when Ed ended up in England and could, y'know, understand what everyone was saying.