A timesink is any endeavor in which time sinks away meaning it takes up time to do.
This term is sometimes derogatory, but often just neutral in tone, and seen usually as a joke or jest. For example, "Timbermaw grinding is such a timesink".
Many people believe World of Warcraft is an addictive timesink.
It is widely referred that reputation grinds are a timesink in the fact that it takes a very long time to achieve what you set out to do. Generally, anything that takes more than a week to do could probably be considered a timesink (apart from the regular leveling-up of a character, which is expected to take a long time).