The word "under" could be linked to the fact that Satan and his followers will always be below heaven. Satan groans inwardly as opposed to outwardly because he does not want to show his pain. If he were to show his suffering and regret, the other angels might lose faith in him as their leader. Satan is getting what he deserves here, because he boasted that he could defeat God. His mistake has forced him to live in his own Hell forever. The word "under" is also used again signifying Satan's position.