Here are a few of my favorite entries, all beginning with the letter “H," from Word and Phrase Origins, a thoroughly delightful book published in 1997.
Half-back. A fairly new term for northern Americans who retire to Florida, find it too hot in the summer and go "halfway back" to settle in North Carolina.
Happy as a clam at high tide. Clams are usually dug at low tide, so this expression makes perfect sense. It was first noted in America in 1834.
Have a bear by the tail. This expression arose in America during the first half of the 19th century. To have a bear by the tail is to be in a bad situation--you're in trouble whether you hang on or let go.
Heckle. First recorded in the 13th century, heckle originally meant to disentangle and straighten flax fibers by drawing them through an iron comb. Since the comb disturbed the fibers, heckle came to mean to annoy or harass someone.
Hepatica. These are little wildflowers with three-lobed leaved resembling the human liver. Someone noticed this way back in medieval times and named them from the Greek hepatikos ("of the liver").
A note about the author: Word and Phrase Origins was written by Robert Hendrickson, now 85 years old and the author of more than 25 other books, including American Literary Anecdotes, New York Tawk, and More Cunning than Man: A Social History of Rats and Men.