Unnatural means not the way nature does it, or not the way a person normally behaves. Grass that green in February, a smile held too long for the camera, silence in a house that is never quiet.
One half of the word points at nature: unnatural heat for October, an unnatural colour in the water, an unnatural bend in the branch. The other half points at people, where it means forced rather than genuine — an unnatural laugh, an unnatural pose, an actor who sounds unnatural reading her own lines. For centuries the word also carried a heavy moral charge, which is why old court reports speak of unnatural acts; that use is dated and best left alone. It survives in one phrase you will meet in the news: died of unnatural causes, meaning killed rather than dead of illness or age. The adverb is unnaturally, and watch the double negative in 'it is not unnatural to feel nervous', which means it is perfectly normal.
1adjectiveunnaturalNot produced by nature; against the ordinary order of nature.
“The lake had an unnatural green tint after the spill.”
2adjectiveforcedForced or false rather than genuine.
“His laugh was so unnatural that everyone looked up.”
synonyms: forced, artificial
3adjectiveuseIn older and dated use, shocking or wicked by the standards of accepted morality.
“The pamphlet called the whole business unnatural and monstrous.”
There is no long journey here: unnatural is un- plus natural, joined in Middle English and unchanged since. Un- is English's oldest way of saying not, and it fastens onto almost anything. The plainness is why the word can mean two things at once — against nature, and not how you would really behave.
Middle English
unnatural
English, today
unnatural
From Middle English unnatural, unnaturel, equivalent to un- + natural.
Etymology from Wiktionary, CC BY-SA