No mystery here, just a strange misprint in Gibbon. At least I’m assuming it’s a misprint as I can make no sense of it otherwise. Here’s the passage, from Ch. LVIII, footnote 77 of the Decline.
William of Malmsbury (who wrote about the year 1130) has inserted in his history (l. iv. p. 130-154) a narrative of the first crusade: but I wish that, instead of listening to the tenue murmur which had passed the British ocean, (p. 143,) he had confined himself to the numbers, families, and adventures of his countrymen.
Now there is a noun tenue in OED (but no adjective) meaning “Carriage, bearing, deportment; also, costume, ‘rig’. Also transf.” but for the life of me I can’t make that fit with the quoted text. I figure that it has to be a misprint for tenuous. If so, I must say it’s a rather odd error for a compositor to make (and it is originally down to a compositor as the text quoted is exactly the same in my 1962 Everyman edition. Earlier editions I can’t speak to). There again printers are only human and if the error dates back to Gibbon’s original MS (although I think that’s unlikely) it’s more understandable. Do you guys agree that this is a misprint?