I don't think I'd heard it before, but it's a genuine exclamation. Definition from "A dictionary of slang and unconventional English" by Partridge&Beale is as follows:
fuck my old boots! A c.p. connoting astonishment: Londoners' (C.20), hence RAF (1918 onwards), then gen. A humorously euph. var is seduce my ancient footwear!
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timeasmymeasure
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Date: 2010-11-06 06:58 pm (UTC)fuck my old boots! A c.p. connoting astonishment: Londoners' (C.20), hence RAF (1918 onwards), then gen. A humorously euph. var is seduce my ancient footwear!