turtleneck means: The American term turtle-neck refers to a polo neck. However, in England where both are used, the name has two meanings. This neckline, which is also tubular in the English is not quite as high as or as folded as a traditional polo neck. This neckline is named after its resemblance to a turtle’s neck as it comes in and out its shell. (in Fashion Dictionary)

What else does turtleneck mean?

An oversized, tube-like collar, which fits close to the neck. (in Merlin Dictionary)

This type of collar is found on a garment such as a sweater. (in Merlin Dictionary)