Some more unknown expressions
bite the bullet -
do or decide to do something very difficult - before the development of anesthetics, wounded soldiers would be given a bullet to bite while being operated on, so as not to scream with pain.
bless you -
customary expression said to someone after sneezing - while there are variations around the theme, the main origin is that sneezing was believed in medieval times to be associated with vulnerability to evil, notably that sneezing expelled a person's soul, thus enabling an evil spirit - or specifically the devil - to steal the soul or to enter the body and take possession of it. Another contributory factor was the association of sneezing with the Black Death (Bubonic Plague) which ravaged England and particularly London in the 14th and 17th centuries.