The story
Named during the Vietnam War, as the lizard's cry could be heard as fuck you. The fuck sound is quite clear and short in duration, followed by a pause of about half a second and the elongated you. The same sound could also be heard as tokay (as in tokay gecko), as tokeh (as in tokeh-tokeh), or as tuctoo, all of which are other English names of the lizard.
Etymology from Wiktionary, CC BY-SA 4.0.