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1.
In close air support, artillery, mortar, and naval gunfire support fires, it is the term included in the method of engagement segment of a call for fire which indicates that friendly forces are within close proximity of the target, usually 600 metres however The close proximity distance can be determined by the weapon and munition fired.
"steelrain, steelrain, this is Hitman calling in fire mission, popeye uniform 059 098 038 degrees 200 metres, danger close"

"roger that hitman, splash in 5 seconds"
by CaptainIncredible June 15, 2009
 
2.
A term that is meant too be shouted when you are in a public toilet and your about to take a massive dump after consuming food such as curry. This technique should be used to warn people or too create tension for no particular reason or for your own personal enjoyment.
You've just taken a dump and you shout "DANGER CLOSE" as loud as you can too warn people of the event. You then casually walk out without making eye contact with anyone else in there.
by ike2011 March 06, 2011
 
3.
Military jargon for a fire mission ordered within close proximity of friendly units. Also to break wind - usually with devastating effect - in the company of close friends or family.
Josh: "Oscar Zulu Yankie Delta Niner, artillery away, danger close." *farts in friend's face*

Dana: "Jim, don't break wind in the room when my mother has come to visit."
Mother: "Since when did Jim care about danger close?"
by Nehima Six66 November 14, 2009
 
4.
Used in military radio traffic to inform some one that the sending unit has enemy contact very near their exact position.
*Marine* Sir, it's so damn dark, I can't see 'em, but I can smell 'em.
*Marine Captain speaking into radio* Echo 2 Charlie be advised we have danger close.
by LawDog1317 August 22, 2007