What is the heaviest organ in the human body?

The liver is the heaviest organ INSIDE the human body.

Taking away the "in" modifier, the skin is the body’s heaviest organ, with a mass of 4-5 kg, and a total surface area of about 1.2-2.2 m2. The skin is composed of two layers, the epidermis and the dermis. The epidermis is the outermost layer, which principally functions as a protective barrier, while the dermis is the vascularised, innermost layer. Beneath the skin, there is a layer of loose connective tissue known as the hypodermis, or superficial fascia.

This layer is not part of the skin itself, but instead it functions to loosely attach the skin to the subadjacent tissue. As a fatty layer, the hypodermis also functions as a shock absorber and as an insulator.

http://www.bioeng.auckland.ac.nz/physiome/ontologies/skin/organ.php

Average organ weights:

1. Skin: 4500g
2. Liver: 1560g
3. Brain: 1500g
4. Lungs: 1300g
5. Heart: 300g

http://www.sciencefocus.com/qa/top-10-what-are-heaviest-organs-human-body