Old English heortleas "dispirited, dejected;" see heart (n.) + -less. In Middle English with expanded senses "lacking in courage; foolish; listless; half-hearted; sluggish." Sense of "callous, cruel, wanting in kindly feeling" is not certainly attested before Shelley used it in 1816. Literal meaning "lacking a heart, lifeless" (mid-15c.) is rare. Related: Heartlessly; heartlessness. Similar formation in Dutch harteloos, German herzlos.
1. It was a heartless thing to do.
2. I couldn't believe they were so heartless.
3. What a heartless thing to say!
4. You couldn't be so heartless as to disown him.
5. The heartless witch cast a spell on the poor little girl.