Although "old man pants" is an alternative, I think this is the better way to express the term.

http://hipstermusings.blogspot.com/2010/04/happiness-means.h...

So I can't help but chuckle when I think about how American Apparel, well-known purveyor of SEX, is now selling unisex grandpa pants. Hilarious as it is, grandpa pants are kinda cool so I'm not complaining. I also tried to be clever with this outfit by pairing a sexy see-through lace top and spectator heels with the utterly un-sexy grandpa pants. Ok, now I need to go settle in with a cup of tea and a blanket and attempt my best efforts at "studying" for "exams". Unfortunately, it will probably result in me watching movies on my laptop. Any suggestions?


--------------------------------------------------
Note added at 1 hr (2010-12-07 14:19:59 GMT)
--------------------------------------------------

Here are websites with pics “ojipan”:
http://www.hepfive.jp/shop/heather/item/201006/post_1876.htm...
http://m-deux.jp/products/2213.html
http://plaza.rakuten.co.jp/haruiku/diary/201001270001/

http://keyword.eruca.jp/w352500/all/1
スタジャン￥2,940、ニットカーデ￥2,940、チェックパンツ￥4,095、手袋￥1,995（ともにwits）TOTAL￥11,970　カットソー＆ブーツ私物　【ブラウン×グレー＝おじいちゃんが着ていそうな組み合わせ（笑）そこに元気な印象のスタジャンを合わせて可愛らしさをプラスしました★】

Here's a website in English using "old man pants":
http://thatraraavis.blogspot.com/2008/10/old-man-pants.html
old man pants
i have been on a hunt for the perfect pair of old man pants for several years now. as far as thrift stores finds go, they rank up there with the always elusive perfect leather jacket. i finally found a pair that are workable. they are obviously too big in the waist, but work well paper-bagged with a belt (without a belt they fall down around my ankles. haha). plus, i really like the shape, they are just baggy enough.
i am still playing around with them though and searching for the perfect top/cardigan/jacket combo to wear on top. so far, i am really pleased with my old guy purchase.