wire
n.— «Those of us who conduct missions “outside the wire’ on a regular basis have come up with a variety of terms to refer to those who remain safe and comfortable on base.» —“Fobbits and other Iraqi critters” by Steve Smith in FOB Bernstein, Iraq ArmySteve Nov. 12, 2004. (source: Double-Tongued Dictionary)
Further reading
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In the thriller Down Cemetery Road starring Emma Thompson, a character uses the Briticism safe as houses, meaning “quite safe,” an expression thought to derive either from the sturdiness of a house or the sense of real estate as a secure investment...