it probably means " long time in position", for example, Australia's longest-serving independent senator Brian Harradine has died in Hobart aged 79 after a long illness.

It does mean "longest time in a position" as Stephen said. The combination of two adjectives like this is often done by putting a hyphen, the <-> , between the words to combine them into one! but it's not always used. A common example would in a phrase like " my one-year-old baby".