This word for a detective owes its origin to the Old Norse "sloth," and later the British "sleuth," both meaning the track of an animal or person. But how did a track become a gumshoe?
For centuries, sportsmen have bred dogs for their tracking abilities. A "sleuthhound" was a dog trained to follow the track (sleuth) of a quarry in all weather. This Scottish bloodhound not only hunted game but also tracked down fugitives.
Over time, the hound gained international fame and became the symbol of any eager and dogged pursuit of an object or person. By the 19th century, the sleuthhound became the epithet for a keen-nosed investigator and was shortened to sleuth.
This expression meaning to retreat or back down dates to the Middle Ages and refers to a common practice of land snails. These creatures withdraw their soft projections, or "horns," inside their shells when they feel threatened by danger or when the weather is poor.
Whoever was watching the snail so closely back then must have seen a resemblance between the snail's retreat and any figurative withdrawal from an unfavorable situation.
SOURCES: 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson; 'Have a Nice Day!' by Christine Ammer; 'The Barnhart Dictionary of Etymology,' by Robert K. Barnhart; 'The Story Behind the Words,' by Morton Freeman; 'Why You Say It,' by Webb Garrison.