냅두다 is the casual squeezed form of 내버려 두다, leaving something or someone alone, and -게 냅두다 is letting them get on with it: 저는 뭐 그냥 보통 다 하게 냅두는 편이에요 'I usually just let them do whatever', 아 그럼 사게 냅둬요 왜! 'then let me buy them!'. It also covers leaving a thing to run its course: 지나간 것은 지나가게 냅둬라, let what's past be past. Write it 내버려 두다 or 놔두다 in anything careful.