g.m.b.u., n. — Green’s Dictionary of Slang

g.m.b.u. n.

also g.m.f.u.

[abbr. grand military balls-up/grand military fuck-up]

a disaster, utter chaos.

1983		J. Mortimer Rumpole and the Golden Thread 187: I must say it’s all a G.M.B.U. [...] Grand Military Balls-up. [Ibid.] 188: That’s what the row was all about, wasn’t it. A Grand Military Shout-up.
1993–2002		Probert Encyc. 🌐 G.M.F.U. (Grand Military Fuck Up) is British slang for a complete disaster.