Other forms: insufferably
If something is insufferable, it's unbearable and impossible, like the insufferable humidity of the "rain forest room" at the zoo on a hot summer day.
You can see the word suffer in insufferable — and it's no coincidence. If it's unacceptable or too much to handle, it's insufferable. An egotistic co-worker is insufferable, and so is a blizzard that leaves six-foot drifts against the doors of your house, or insufferable working conditions in many of the factories that produce goods around the world.