I thought this might be a more interesting title than "the etymology of modern day idioms." Anyway I find the topic interesting.

"Burying the hatchet" or putting aside differences apparently derives from the natve american custom of putting out of site all weapons before smoking the peace pipe.

"Close but no cigar" meaning to almost win dates back to the time in the US when early slot machines used to give cigars as prizes.

"Cat got your tongue" making to be unable or unwilling to speak. Well there seems to be a couple of schools of thought on this one. One thought is that the expression comes from the supposed ancient traditional middle-eastern practice of removing the tongues of liars and feeding them to cats.

"Mad as a hatter", This one is self evident but dates back to the time when mercury was used in the construction of felt hats. Mercury exposure can cause aggressiveness, mood swings, and anti-social behaviour.

Does anyone have any more favourites?