As defined in the Preface, an "idiom" is a new meaning assigned to a word or phrase that already has its own meaning. English is heavily idiomatic, an observation the editors try to explain while pointing out that all known languages (there are many yet to be explored) have idioms.
The work describes more than 4000 idioms with definitions, usage, and by parts of speech. The types are described in the Preface: Lexemic idioms reflect the parts of speech: verbal, nominal, adverbial, adjective. Others are identified by size and form: Tournures are the longer phrases ("seize the bull by the horns"), and some are "frozen" or inflexible. (You are not likely to be understood if you say "the bucket has been kicked by the cowboy".)
Finally, the book includes established "sayings and proverbs" which are idiomatic: "don't wash your dirty linen in public", and "don't count your chickens before they hatch".
Although Adam Makkai is old enough to know, he defines "big lie" in relation to Watergate, rather than to King Leopold of Belgium, whose use of it was eulogized by Joseph Goebbels. This illustrates the Editor's choice of current illustration rather than etymological reference. For some reason, perhaps peculiar to idiomography, the editors are INTENT upon clear understanding. They spell out the meanings with illustration, usages, and context. This further reflects their belief that "the use of idioms is...extremely important. It can strike a chord of solidarity with the listener." [vii] This certainly does: Struck, the chord is. ( )