This is a polite way of referring to having sexual relations with her. See how you translated a similar phrase in [Genesis 16:2](../16/02.md). (See: [[rc://en/ta/man/jit/figs-euphemism]])
You may need to make explicit that the word "it" refers to Onan's semen. Alternate translation: "he destroyed his semen by having it flow on the ground" or "he did not send his semen into her" (See: [[rc://en/ta/man/jit/figs-explicit]])
# on the ground
This is probably a euphemism for any place that it should not have gone. (See: [[rc://en/ta/man/jit/figs-euphemism]])