0 to fasten something together using string, rope, or something similar: --
It was an old-fashioned parcel, tied up with string.
1 to cause something, often money or possessions, not to be available for use: --
2 if money is tied up, it is invested in something and not available for spending on something else: --
3 to agree all the details of a business arrangement: --
4 to use a piece of equipment so that it cannot be used by anyone else, usually for a long time: --