Causing heartsickness; deeply distressing or emotionally upsetting in a way that makes one feel sick at heart.
From 'heartsick' plus the suffix '-ening,' which forms adjectives meaning 'causing or tending to cause' (as in 'frightening,' 'sickening'). This creates an adjective describing what causes the emotional state.
The '-ening' suffix is productive in English—it lets us turn any state or feeling into a description of what causes it—so 'heartsick' becomes 'heartsickening,' just as 'dark' becomes 'darkening' or 'sick' becomes 'sickening.'
Complete word intelligence in one call. Free tier — 50 lookups/day.