overconfidence

What it means

1. uncountableusuallyAn excessive or unwarranted degree of confidence.

   My case is, as I have told you, almost complete; but we must not err on the side of over-confidence.

   1890 February, A[rthur] Conan Doyle, “Sherlock Holmes Gives a Demonstration”, in The Sign of Four (Standard Library), London: Spencer Blackett […], →OCLC, page 89

   1 more example

   Yet United may have paid the price for overconfidence as they completely lost their concentration after 10 minutes to let Wolves back in.

   2011 May 2, Chris Whyatt, “Wolverhampton 2 - 1 Man Utd”, in BBC

In a sentence

My case is, as I have told you, almost complete; but we must not err on the side of over-confidence.

1890 February, A[rthur] Conan Doyle, “Sherlock Holmes Gives a Demonstration”, in The Sign of Four (Standard Library), London: Spencer Blackett […], →OCLC, page 89

Where it comes from

1. Proto-Indo-European*úp
2. Proto-Indo-European*upér
3. Proto-Germanic*uber
4. Old Englishofer-
5. Middle Englishover-
6. Englishover-
7. Englishconfidence
8. Englishoverconfidence

From over- + confidence.