OMD! It's Word and Phrase Derivation Day!

Oh, my dog! We're gonna have fun. Here's a fine selection of word and phrase origins. Enjoy! 

To beat the living daylights out of someone. To say "I'll let daylight into you!" to an enemy in day's past was to threaten that you'd open him up, make a hole in him with a sword, knife, or gun. The expression, in the form of its variant "I'll make daylight shine through you" is recorded in America as early as 1774 and is probably much older. Sayings like "I'll fill him full of holes" replaced the older expression when modern weapons like machine guns made wholesale ventilation easier, but it lived on in the form of "I'll beat the living daylights out of you." Unlike the old swordsman's words, this makes no sense literally. It is merely the ghost of an imaginative phrase. 

Echo. The Greek nymph Echo talked so much that Zeus and his wife Hera couldn't hear what any of the other nymphs were saying. Hera punished this nymph by depriving her of all speech save the ability to repeat the words of others, giving her heartaches -- she lost her love, Narcissus -- but giving us the word "echo". 

Left-handed compliment. The expression left-handed compliment, "a thinly disguised insult that poses as praise", apparently has its origins in the practice of morganatic marriage, widely prevalent among German royalty in the Middle Ages and even practiced in modern times -- Archduke Ferdinand and his wife, who were both killed in the assassination that set off World War I, were married morganatically. These were usually marriages between royalty and commoners, the commoner agreeing that she would have no claim to her royal husband's title or property, nor would any children of the marriage -- all she received was a morning gift (from the Latin morganaticum) on the morning after the marriage was consummated. In the special wedding ceremony held for these marriages, common up to the 17th century, the groom gave the bride his left hand instead of his right, and thus morganatic marriages came to be known as left-handed marriages. Since they were a thinly disguised insult, it is possible that they later lent their name to the deceptive left-handed compliment.

As usual, the above is taken from "The Facts on File Encyclopedia of Word and Phrase Origins" by Robert Hendrickson.