The small tugboat is slowly pulling the huge tanker into the dock.

tugboat 是复合名词，由 tug（拖、拉）和 boat（船）组成。tug 来自动词"用力拉"，boat 指"船、艇"。tugboat 指专门用来拖曳其他船只的小型动力船，即"拖船"。