Sinister
Latin for "left", achieved its negative connotations through the fact that augers (priests who would interpret the will of the gods through studying the flight of birds) used to look to the North for omens, and considered anything which appeared in the West - on their left - to be bad luck.
Following this, if a Roman cursed you while pointing with his left hand, it was considered worse and more final than by doing it with his right.
Blakmail
Its origin had nothing to do with the post office. Mail in this sense was an old Anglo-Norse term for rent or tribute. During the time of border warfare between England and Scotland, freebooters extorted payment from farmers of the area in exchange for protection and immunity from plunder. As the inhabitants were generally very poor, the tribute was paid in "black mail," that is, grain, meat, or the lowest coinage (copper), as opposed to "white mail," which was silver. In time the word took on the meaning of any payment extorted by threat of exposure of an incriminating secret.
Any more?