Why is 'sanguin(e)' in French opposite in meaning to 'sanguine' in English

Question

My French colleague was confused when I described someone as sanguine about not receiving a promotion. Upon investigation it seems that the English meaning (to be cool in the face of adversity) is exactly opposite to the French (to be hot-headed / angry).

Any ideas what led to this divergence in meaning, even though they clearly come from the same root (sanguin being Latin for blood)? The English definition is actually more confusing given the connotation of hot bloodedness / red with anger or passion!

Answer 1

The prime meaning of the word sanguin when used to qualify people's temper in French is the one inherited from Hippocrates's classification:

Sujet (...) qui (...) se présente comme un individu massif, au visage empourpré; ses réactions sont rapides: il est généralement sociable et optimiste, mais aussi emporté et violent¹ (Sill. Psychol. 1980).(TLF)

In English we get the following definition from the OED:

Having the mental attributes characteristic of the sanguine complexion [...]; chiefly, disposed to hopefulness or confidence of success¹.

If we consider these highly trustworthy dictionaries I'm afraid saying that the French adjective sanguin is "opposite in meaning" to the English "sanguine" is not really appropriate.

What is true is that French still uses sanguin in a figurative way to mean "violent" whereas in English it is considered to be archaic:

2. Archaic a. Having blood as the dominant humor in terms of medieval physiology.
   b. Having the temperament and ruddy complexion formerly thought to be characteristic of a person dominated by this humor; passionate. (American Heritage Dictionary)

In 21st century English the word "sanguine" has sometimes been used to mean "at ease; accepting", a meaning that won't be found in my 20th century OED or Webster's dictionaries but that can be found in contemporary use (e.g. 2004 quote). FL is not the proper place to dig further into how the word has taken on this contemporary meaning in English, but anyone interested could go to English Language & Usage: Just how is the the adjective "sanguine" used in current English?

^{1 My emphasis.}

Answer 2

It may be a matter of temperature.

The English are said to be "cold blooded" (the famous "flegme" britannique) while the French, and more generally Latin people (Spanish, Italians, and French) are said to be "hot blooded."

However, all these differences need to be more deeply understood, though it stays true, in my belief, that an English person will not lose his balance as easily as a French person might do, under certain circumstances.