Here are a few of my favorite entries, all beginning with the letter “I," from Word and Phrase Origins, a thoroughly delightful book published in 1997.
Iceblink. First recorded in the late 18th century, iceblink is the name seamen gave to the luminous sky caused by light reflected from ice.
Imp. The Anglo-Saxons called a graft or shoot an "impian." Shortened to imp, it later came to mean an offspring or a child. Later still, it came to refer to not any child, but a mischievous one.
In like Flynn. Edward "Boss" Flynn's Chicago political machine of the first half of the 20th century never lost an election, and was always "in office," inspiring the expression in like Flynn. It is now used in the broader sense of "to have it made."
In the pink. First recorded in 1905 as "in the pink of condition," in the pink refers to the healthy pink of firm flesh.
It's not what it's cracked up to be. To "crack" was standard English for "to boast or brag" until about 1700. Today, this sense of the word is only found in this expression which means, of course, that something is not as good as people say.
A note about the author: Word and Phrase Origins was written by Robert Hendrickson, 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.