Etymology From ab- + normal. First attested in 1835, replacing the earlier anormal and even earlier abnormous, from Latin abnormis (“departing from normal”), from either ab- (“away from”) + norma (“rule, norm”), or Ancient Greek ἀνώμαλος (anṓmalos).
1
Not conforming to rule or system; deviating from the usual or normal type.
And then after an abnormal meal, which was either a very late breakfast or a very early lunch, they drove on to Victoria Station.1899, Arthur Conan Doyle, chapter 6, in A Duet:
Here perhaps one of you might interrupt me and say, 'But what is it that you do, Socrates? How is it that you have been misrepresented like this? Surely all this talk and gossip about you would never have arisen if you had confined yourself to ordinary activities, but only if your behaviour was abnormal. Give us the explanation, if you do not want us to draw our own conclusions.'1993, Plato, translated by Hugh Tredennick and Harold Tarrant, “Justice and Duty (i): Socrates Speaks at his Trial: the Apology”, in The Last Days of Socrates (Penguin Classics), revised edition, London; New York, N.Y.: Penguin Books, →ISBN, page 41:
2
Of or pertaining to that which is irregular, in particular, behaviour that deviates from norms of social propriety or accepted standards of mental health.
Furuseth was right; I was abnormal, an "emotionless monster," a strange bookish creature, capable of pleasuring in sensations only of the mind.1904, Jack London, chapter 23, in The Sea-Wolf (Macmillan’s Standard Library), New York, N.Y.: Grosset & Dunlap, →OCLC:
Many of the so-called rites of these secret societies were so patently ridiculous, that it is quite obvious that they were merely an excuse for men and women to indulge in sex-play and lustful gratification, frequently of an abnormal kind.1936, Rollo Ahmed, The Black Art, London: Long, page 161:
Etymology From ab- + normal. First attested in 1835, replacing the earlier anormal and even earlier abnormous, from Latin abnormis (“departing from normal”), from either ab- (“away from”) + norma (“rule, norm”), or Ancient Greek ἀνώμαλος (anṓmalos).