The following two expression have the same meaning and the first appears to be typically used BrE while the second in AmE: swear blind (british) or swear up and down (american) :
If someone swears blind that something is true, they insist that they are telling you the truth.
He swore blind that he hadn't taken the money. He swears blind that he bears no grudges against Manchester United for sacking him, but I don't know if I believe him.
(Collins Dictionary)
According to the site theanswerbank.co.uk the "earliest recorded use of the phrase 'swear blind' is from John Fowles' novel, The Collector, published in 1963", but as evidenced by Google Books there are much earlier usages of the expression which date back to the beginning of the 20th century.
Early usage exemples:
From Rickerby's Folly by Tom Gallon 1905 –
if I was to set 'em digging up the body, ten chances to one but what they'd go an' swear blind I'd murdered 'im, an' buried 'im meself
From The Fast Gentleman by Keble Howard – 1928 –
I shall swear blind I never touched their old boat ! An' I didn't, neever. I on'y cut the ropes an' let 'em go. An' a good riddance, I sh'd sye !
I couldn't find details about their origin which is probably linked to the meaning, maybe obsolete, of blind and up and down.
Can anybody provide more information about these two expressions and their origin?
Best Answer
As far as UK and US popular press evidence goes, for 'swear blind', use of the verbal phrase, where 'blind' modifies the action 'to swear', is found as early as 1886 in the UK, and 1909 in the US. Use of the expression was much more common in the UK, at least as found through the first decade of the 20th century.
Derivation of the verbal phrase seems to be from emphatic shortening of common earlier set phrases with adjectival use of 'blind' modifying [something] (rather than modifying the verb 'to swear'): "swear blind [obedience, allegiance, fealty, etc.]". The adjective "blind" in the fuller expressions was sometimes paired with another adjective; for example, "swear blind and unreasoning obedience".
The reason 'swear blind' is more common in the UK, and appeared so much earlier in the popular press there, may be sociological. As speculation, while a tradition of blind, unreasoning loyalty is an established cultural feature of any kingdom, that same tradition has less force in the more individualistic US, particularly after the revolution.
The phrase 'swear up and down' in early use always involved either [something] sworn to, as in "swear [something] up and down" or "swear [something] up or down", or it involved motion, as in "he swore up and down the veranda".
For example, the earliest use of "swear up and down" I found in the popular press took the form of "swear up and down [to somebody] [that something is true]":
The use of 'to swear up and down' or 'to swear up or down', if it does not derive from the motion of the swearer (as in the earlier-mentioned "swear up and down the veranda"), derives from a peculiar practice once (apparently) only too common. That practice is described in an early diatribe:
Initially, the phrase with reference to the practice of swearing [something] "up or down" was only "up or down"; it later developed into swearing [something] "up and down":
Early uses of 'swear up and down [that something is or is not true]', or 'swear up or down [etc.]', in the US included uses referring to paid perjury, just as in the UK. For example,
A vestigal sense of swearing something up or down (true or false) remains in the set phrase "swear up and down"; however, the more influential derivational sense (although still vestigal) seems to be the sense of 'swearing-in-motion', as in "he would advise any slave to swear, up and down, that his master was a secessionist" (The Liberator, Boston, Massachusetts, 24 Jul 1863; paywalled). That this is the more prominent derivational source is especially evident in a later elaboration of the phrase, wherein things are sworn "up, down and sideways" (mid-20th century).
I speculate that the reason 'swear up and down' appears more frequently in the US may be that the sense of 'swear up or down', referring to paid perjury, was less common in the US, and was associated in the US with European practices (see quote). Thus, the development and adoption of the phrase 'swear up and down' encountered less interference from the conflicting sense of 'swear up or down' in US use than in UK use.