An accumulation caused by clogging or a stoppage. "He discovered a backup in the toilet"
2.
Someone who takes the place of another (as when things get dangerous or difficult). Synonyms:backup man, fill-in, relief, reliever, stand-in, substitute."We need extra employees for summer fill-ins"
... "The girl at Karry Karton thought about talking to me, and I heard my phone ring, even though the bell was disconnected. G.G. thought about needing backup material for the conference and I went to the library. The truck driver thought about warning people and I got out of his way. So I can read people's minds—some people's minds, some of the time, anyway ... only there's no such thing as telepathy. And if I'm not telepathic, then...." ... — The Sound of Silence • Barbara Constant
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