wiktionary: Named "Fuck-you lizard" during the Vietnam War, as the lizard's cry sounds like "fuck you." The "fuck" sound is quite clear and short in duration, followed by about a 1/2-second pause and the "you" elongated.
I read once in Tumblr's "Did You Know?" that the American troops nicknamed that lizard the "Fuck You Lizard" because it seemed to be saying that So as the AmeViet shipper that I am, I imagined Vietnam gave one of those lizards to America since he seemed to like it so much. She seems not to get the joke though
The lizard specifically is a gecko called the Tokay gecko