This article is from the Puzzles FAQ, by Chris Cole chris@questrel.questrel.com and Matthew Daly mwdaly@pobox.com with numerous contributions by others.
What are some examples of idioms that include obsolete words?
language/english/etymology/fossil.s
These are called fossil expresions -- words that have dropped out of
common use but hang around in idioms. Not all of them are separate
words, some are part of other words or have prefixes or suffixes
attached. There are also words which have current meaning, but the
meaning in the idiom is unrelated to it.
idiom fossil meaning of fossil -------------------------------------------------- swashbuckler buckler small shield newfangled fangled siezed rank and file file column to and fro fro from gormless gorm attention hem and haw haw make the sound "haw" hem and haw hem make the sound "hem" hue and cry hue outcry kit and kaboodle kaboodle collection out of kilter kilter order kith and kin kith friends let or hindrance let hindrance footpad pad highwayman pratfall prat buttocks rank and file rank row raring to go raring enthusiastic ruthless ruth compassion short shrift shrift confession spick-and-span span chunk of wood spick-and-span spick nail (spike) swashbuckler swash bluster or stagger bank teller tell to count
 
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