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To firefighters (mostly) an incident, situation or place that is or could be a dramatic rescue, or is reminiscent of a rescue on the show Emergency!, which featured Squad 51.
(Upon looking at a factory machine room full of pipes) "This place is so Squad 51!

"That extrication today was pure Squad 51 stuff."
by Ironsman6 January 20, 2004
An exclamation, from the 70s show Emergency.

It comes from the fire department dispatch guy that goes "Squad 51!"
"(random DTMF tones) Squad 51! (insert random dramatic event here)

"roger, squad 51, KMG365"
by IrishRepublicanArmy November 28, 2003