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Webster 1913 Edition

Noun.

An animal, particularly a log, having an uncut tail. Cf.

☞ A longtail was a gentleman’s dog, or the dog of one qualified to bunt, other dogs being required to have their tails cut.

Cut and longtail

all, gentlefolks and others, as they might come.

Shak.

See also: long tail

longtail ‎(plural longtails)

A common designation for an animal that has an unusually long tail relative to similar species.
In Bermuda, an alternative name for the white-tailed tropicbird.
A longtail boat.
(colloquial) In the Isle of Man, a rat.

The colloquial usage derives from a relatively modern superstition that it is considered bad luck to use the word "rat".