A.Word.A.Day's word for this day, which is, in the United States, Presidents' Day. Anu, the fount of wisdom at A.W.A.D., defines sockdolager (sok-DOL-uh-juhr) as a decisive blow or remark, or something exceptional or unique, just as dannye says.
But the story does not end here. Apparently, on a fateful April 14 in 1865, Abraham Lincoln was attending a performance of "An American Cousin" at the Ford Theatre in Washington DC. John Wilkes Booth, an actor himself, knew the play, and waited for the line that got the biggest laugh - "Well, I guess I know enough to turn you inside out, you sockdologising old man-trap." - to fire his deadly shot. The noise of the audience's mirth covered the sound of his gun, and the assassin escaped.
Anu goes on to relate that Americans used to have public holidays on the anniversaries of the births of both Lincoln - February 12, 1809 - and George Washington - February 22, 1732. In time, however, the average of the two dates was chosen, and a single day was marked to commemerate not just those two, but all US presidents. Presidents' Day is the third Monday in February, thus giving my neighbours to the south a nice long weekend in this drab and cold winter month.