심쿵 is the abbreviation of 심장이 쿵쾅댄다. So, when sombody 심쿵했다, it means the person's heart ponded especially because the person is attracted to another person. 심쿵"당"했다 is a similar meaning, but more emphasizing that it is not voluntay but involuntary response because of another person's attraction. But anyway, 심쿵 is a recently used slang.