Onomatopoeic. The “large breasts” sense is reborrowed from Japanese ボイン, in turn from English boing.
Meanings
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:
(in the context of anime and comics, slang) A representation of the sound of something bouncing. ▸ Announcing the presence of large, pert breasts, which are bouncy.
(slang) A representation of the sound of something bouncing. ▸ Announcing the advent of male arousal (erection), via the metaphor of a spring springing energetically.