I wrote:

 >> "Arigatou", for example, apparently comes from the Portuguese
 >>  "obrigado", even though it is written in hiragana.

Paul Blay commented:

> Ha-ha, you're such a kidder.
> 
> ... You _are_ such a kidder, right?

Not at all. There's some controversy about it, but the word does seem to 
have come into the Japanese language at about the time the Portuguese 
arrived (1542). See, e.g.:

http://www.ling.ed.ac.uk/linguist/issues/12/12-1871.html

John
http://rarebooksinjapan.com