un•nat•u•ralʌnˈnætʃ ər əl, -ˈnætʃ rəl(adj.)

1. contrary to the laws or course of nature.

2. at variance with the character or nature of a person, animal, or plant.

3. at variance with what is normal or to be expected.

4. lacking human qualities or sympathies; monstrous; inhuman.

5. not genuine or spontaneous; artificial or contrived:

   a stiff, unnatural manner.

6. Obs. lacking a valid or natural claim; illegitimate.

Origin of unnatural:

1375–1425

un•nat′u•ral•ness(n.)