Anything which takes up a lot of your time. I can be homework, a diy project, a tv show, or even a ridiculously long nap. It can be enjoyable, or even productive, but it requiers sinking a lot of time into it... therefore timesink.
I decided to start baking my own bread... didn't realise it was such a timesink.
Mad Men is great, but it is such a timesink.