Other forms: heartrendingly
Something heartrending is heartbreaking: it causes grief and sadness.
The heart is the organ associated with emotions, and to rend something is to tear it, so heartrending things tear up your heart: not literally, but because they make you sad. The death of a friend or loved one is heartrending. Getting a terrible disease like cancer is heartrending. A tragic story can be heartrending if it makes you feel for the people involved. Anything that makes you cry is probably heartrending.