One blogger defined it thus:
"Tatamisé", c'est quand un étranger commence à se comporter comme un Japonais. Il enlève ses chaussures quand il rentre chez lui, il met des chaussons, il dort dans un futon, il mange avec des baguettes, il réfléchit avant de parler, etc.which I'd call common courtesy or common sense, as per. It's a big deal getting a bed into yer average Japanese apartment, and it's usually a small bed. I had one at the dorm-- a twin-- and couldn't turn over in it unless I was extreeemely careful, whereas my futon allowed for as much rolling as I wished in the old house in Nakano-ku.
(It's when a foreigner starts acting like a Japanese- they take their shoes off when they come home and put on slippers, they sleep in a futon, they eat with chopsticks, they think twice before speaking)
The Nihonjin mo shiranai Nihongo woman cites examples of people who've remained tatamisé after returning home-- waiting for cab doors to open automatically (expat, I snort: who takes taxis regularly in Japan?), giving aizuchi even in their own language (well, yes, I do) or or moaning about their katakohri (frozen shoulders) and searching vainly for a heat plaster to put on them. 'The concept of katakohri doesn't exist outside Japan', she says, which I suppose is true. I'm more likely to say My neck hurts, even though it's the shoulder blades. But yay for living in a Chinese diaspora city: I've always had access to heat plasters. And in Koreaville, I have access to Salonpas. (Of course, Bengay works better for everything.)
(She gives that other classic cultural clash, the Chinese guy who sees a building with 湯 on the noren and people going inside it with bowls, and figures it's a take-away soup place where you bring your own container. The kanji means hot water in Japanese and is used on bath houses, but it means soup in Chinese.)