Etymology Onomatopoeic. The “large breasts” sense is reborrowed from Japanese ボイン, in turn from English boing.
1
A representation of the sound of something bouncing.
"Phut." Something whipped into the ground beside him and there was a pinpoint flash from the cabin. "B-o-i-n-g-g-g." There was another flash and the bullet hit the rail and whined off into the night.1956, Ian Fleming, Diamonds are Forever, published 1965, page 100:
2
LifestyleSlang Announcing the presence of large, pert breasts, which are bouncy.
In the context of anime and comics
3
Slang Announcing the advent of male arousal (erection), via the metaphor of a spring springing energetically.