swear
swear black is white vigorously maintain anything, however unlikely, in order to get what you want.
swear blind affirm something in an emphatic manner. British informal
☞ A North American variant of this expression is swear up and down.
swear like a trooper swear a great deal.
☞ A trooper was originally a private soldier in a cavalry unit. Troopers were proverbial for their coarse behaviour and bad language at least as early as the mid 18th century: in Pamela (1739–40), Samuel Richardson writes 'she curses and storms at me like a Trooper'. Compare with lie like a trooper (atlie).