(military) Friendly fire.

(countable and uncountable) The colour of the clear sky or the deep sea, between green and purple in the visible spectrum, and one of the primary additive colours for transmitted light; the colour obtained by subtracting red and green from white light using magenta and cyan filters; or any colour resembling this.

(transitive, slang, dated) To spend (money) extravagantly; to blow.

Having blue as its color. / (informal) Depressed, melancholic, sad. / (health care) Having a bluish or purplish shade of the skin due to a lack of oxygen to the normally deep red blood cells.

(ergative) To make or become blue; to turn blue. / (transitive, metallurgy) To treat the surface of steel so that it is passivated chemically and becomes more resistant to rust.

A surname from German. An anglicization of German Blau. / (rare) A female given name from English, typically used in conjoined names like Bonnie Blue or Blue Bell.

A letterman at Oxford or Cambridge. / (historical) A member of the Royal Horse Guards (which merged with the 1st Dragoons in 1969) / Synonym of British Blue (“a breed of cat”)

An indigo material used as a laundry aid to counteract yellowing

present participle of blue

(singular or plural, informal) One's particular life experience, particularly including the hardships one has faced.