Middle English: from Old French curios, from Latin curiosus 'careful', from cura 'care'. sense 2 dates from the early 18th century.

• The word curious came into the language in the sense ‘eager to know or learn something’. Its source is Latin curiosus ‘careful’, from cura ‘care’. The word has had a variety of meanings over the centuries, including ‘skilfully made’, ‘very accurate or precise’, and ‘having an exquisite taste’. The sense ‘strange or unusual’ appeared early on in the 18th century. Among booksellers curious used to be a euphemistic term for erotic or pornographic works.

  The Curious Incident of the Dog in the Night-Time is the title of a best-selling 2003 novel by Mark Haddon but the words come originally from one of Sir Arthur Conan Doyle's Sherlock Holmes mysteries. In the story Silver Blaze (1884), Holmes draws Watson's attention to ‘the curious incident of the dog in the night-time’. When Watson protests that ‘the dog did nothing in the night-time’, Holmes responds: ‘That was the curious incident.’ The point is that the dog did not raise the alarm because he already knew the person who had disturbed him. The saying curiosity killed the cat is first recorded around 1900. The older form is care killed the cat, which is first recorded in Ben Jonson's 1598 play Every Man in His Humour. Curiouser and curiouser is a quotation from Lewis Carroll's Alice in Wonderland (1865). ‘ “Curiouser and curiouser!” cried Alice (she was so much surprised that for a moment she quite forgot how to speak good English).’