A natural bobtail is an animal's tail which due to a mutated gene grows unusually short or is missing completely. The genes for the shortened tail may be dominant or recessive.

What does bobtail mean in trucking?

A semi-truck is in “bobtail” mode when it does not have a trailer attached. Truck drivers often drive a bobtail truck when they are on their way to pick up their cargo at the beginning of a shift, or after dropping their cargo off at the end.

Why do they call it bobtail?

Pierpont refers to a "Bobtail" as the horsetail cut down to avoid entangling the tail in the horse-drawn sleigh. The term is also said to originate from the breed of cats that have short tails. A semi-truck without a trailer looks somewhat similar to these short-tailed cats.