A statement made to signify that what you speak is 100% the truth, the whole truth and nothing but the truth. Usually used when someone doesn't believe what you say, as it is bullshit, but this is used to try and make that person believe its true as its
probably not.
What it actually means may
derive from the old saying of "I swear on
my mum's life!" which means absolutely nothing but some people think if that statement is said that they can't be lying.
"I swear down your honour,
i didn't do it"
Nick: Did you know that
Ahmed got arrested last night?
Keane: No he didn't.
Nick: Yeah man, swear down.