Something that's trunkless has no body or main stem. A statue of two human legs with no torso is trunkless.
A marble sculpture of a person's disembodied head can be described as trunkless because the head is not attached to a body, or trunk, which is defined as "the torso" or "the main part." Trees have trunks, too, which are the central, woody stems to which branches are attached like appendages. Trunkless plants, like most herbs, ferns, and grasses, don't have this main, central structure, but are instead composed of leaves or multiple thin stems.