The story
Coined by Amory Lovins, chairman of the Rocky Mountain Institute as a contraction of negative watt on the model of similar compounds like megawatt, by surface analysis, nega- + watt. Lovins credited the term to a typo in a document by the Colorado Public Utilities Commission in which the word "megawatt" was misspelled.
Etymology from Wiktionary, CC BY-SA 4.0.