1. The hue of that portion of the visible spectrum lying between green and indigo, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 490 nanometers; any of a group of colors that may vary in lightness and saturation, whose hue is that of a clear daytime sky; one of the additive or light primaries; one of the psychological primary hues.
a. A pigment or dye imparting this hue.
a. An object having this hue.
b. Dress or clothing of this hue: The ushers wore blue.
a. A person who wears a blue uniform.
b. blues A dress blue uniform, especially that of the US Army.
5. often Blue
a. A member of the Union Army in the Civil War.
b. The Union Army.
6. A bluefish.
7. Any of various small blue butterflies of the subfamily Polyommatinae.
a. The sky.
b. The sea.
adj. blu·er, blu·est
1. Of the color blue.
2. Bluish or having parts that are blue or bluish, as the blue spruce and the blue whale.
3. Having a gray or purplish color, as from cold or contusion.
4. Wearing blue.
5. Being a trail, as for skiing, marked with a sign having a blue square, indicating an intermediate level of difficulty.
6. Relating to or being a blue state.
a. Gloomy; depressed. See Synonyms at depressed.
b. Dismal; dreary: a blue day.
8. Puritanical; strict.
9. Aristocratic; patrician.
10. Indecent; risqué: a blue joke; a blue movie.
tr. & intr.v. blued, blu·ing, bluesIdioms:
To make or become blue.
blue in the face
At the point of extreme exasperation: I argued with them until I was blue in the face.
into the blue
At a far distance; into the unknown: spontaneously take a trip into the blue.
out of the blue
1. From an unexpected or unforeseen source: criticism that came out of the blue.
2. At a completely unexpected time: a long-unseen friend who appeared out of the blue.
[Middle English blue, bleu, from Old French bleu, of Germanic origin; see bhel-1 in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.