blue

Concise Oxford English Dictionary © 2008 Oxford University Press:
blue1

▶adjective (bluer, bluest)

• 1 of a colour intermediate between green and violet, as of the sky or sea on a sunny day.

  ■ (of a cat, fox, or rabbit) having fur of a smoky grey colour.

• 2 informal melancholy, sad, or depressed.

• 3 informal (of a film, joke, or story) with sexual or pornographic content.

• 4 Brit. informal politically conservative.

• 5 (of a ski run) of the second-lowest level of difficulty.

▶noun

• 1 blue colour, pigment, or material.

  ■ (the blue) literary the sky or sea, or the unknown.

  ■ another term for bluing.

• 2 Brit. a person who has represented Cambridge University or Oxford University in a particular sport in a match between the two universities.

• 3 used in names of small butterflies with predominantly blue wings. [Polyommatus icarus (common blue) and related species.]

• 4 Austral./NZ informal a nickname for a red-headed person.

• 5 Austral./NZ informal an argument or fight.

▶verb (blues, bluing or blueing, blued)

• 1 make or become blue.

• 2 chiefly historical wash (white clothes) with bluing.

– phrases
blue on blue Military denoting or relating to an attack made by one's own side that accidentally harms one's own forces: blue-on-blue incidents. [from the use of blue to indicate friendly forces in military exercises.]
once in a blue moon informal very rarely. [because a ‘blue moon’ is a phenomenon (due to atmospheric dust) that occurs only very rarely.]
out of the blue (or out of a clear blue sky) informal without warning; unexpectedly.
talk a blue streak N. Amer. informal speak continuously and at great length.

– derivatives
blueness noun.

– origin ME: from OFr. bleu, ult. of Gmc origin.

Concise Oxford English Dictionary © 2008 Oxford University Press:
blue2

▶verb (blues, bluing or blueing, blued) Brit. informal, dated squander or spend recklessly.

– origin C19: perh. a var. of blow1.