British slang, interjection, exclamation of surprise short for "God's truth".
From Monty Python's Flying Circus Episode 1: Whither Canada?

Mozart: Thank you, Eddie. And now time for this week's request death. (taking card off piano) For Mr and Mrs Violet Stebbings of 23 Wolverston Road, Hull, the death of Mr Bruce Foster of Guildford.
Cut to a lounge setting. Mr Foster sitting in chair.
Foster: Strewth! (he dies)
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An Australian, or "Aussie", exclaimation, similar to the somewhat more popular "Crikey!"
Strewth, that was a hard day, toss me a Fosters mate!
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Cockney East End abreviation of "It's the truth".
"Strewth Guv would I lie to you?
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An Australian, or "Aussie", slang term, similar to the somewhat more popular "Crikey!"
Strewth, why is it that everybody thinks we Aussies drink Fosters. I wouldn't even feed that to my dog.
by Cock-juggling-thunder-cunt November 24, 2012
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An Australianism or Aussieism for "bloody hell" or "Geeze" or "holy cow", is usually an expression of surprise or disgust or amazement.
G'day mate, strewth you had us all bloody scared when we saw you with that bush pig in the car.
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a British saying when in utter shock, confusion or shocked disappointment
if a jumpscare appears in a movie you could shout " GOD STREWTH MAN !"
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