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.
1) All this homework is making me timesick; I wish we could just go back to kindergarten when our hardest assignment was coloring inside the lines.
2) I'm timesick. I wish I could just fast forward to after graduation.
Something that takes a lot of your time and effort in order to be completed. Fequently used in MMORPGS to make customers play longer, thus making them pay more. Time sinks are very, very, very, very, very bad.