expression, ichidan verb
attack a routed enemy; kick someone when they're down; rub salt in the wound
Literal military sense of pursuing and striking a fleeing enemy; figuratively, adding further harm or criticism to someone already in a bad situation.
敵が逃げるところに追い打ちをかけた。
They attacked the fleeing enemy.
失業した上に病気になるなんて
Losing his job and then getting sick is like rubbing salt in the wound.