Having a boner. It's derived from Taiwanese online forum slang. GG = penis, ininder = getting hard.

the whale tail is the shape formed when a g-string rides up high over a womans trousers

A popular slang used in Taiwan PTT Bulletin Board System, which means that a man's penis expands and holds the erection. It mimics the sounds of traditional Chinese words "雞雞硬硬的" and the letters can be large or small, such as "ggininder".