「Wood or tree」with 「mating display, person palming prick」referring to some sort of a shape that has a main trunk of actual wood, or, resembles a tree trunk, with what appears to be erect penises growing out of it in all directions: a SPINAL COLUMN, also meaning an individual VERTEBRA, suggestive of「having backbone enough」to engage in challenges to battle for copulation rights with females looking on nearby. That same thought and shape applies to a MALLET used to pound something, perhaps the craniums of any and all rivals, or one's prick. And to a species of MUSHROOM, the SHIITAKE in Japanese which propagates in large numbers from the sides of standing but dead shii tree wood (a beech oak species) as it decomposes.
Some suggest「wood, tree with bird plumage」and「wood peckers」refers to the sound of a MALLET made by a WOODPECKER, a bird that nests only in standing but dead trees, normally also full of termites, the woodpecker's favorite snack…and shii mushrooms also only grow on the dead, decaying wood, sprouting from the holes pecked by woodpeckers. Woodpecker plumage is outstanding, normally a stunning combination of bright red, jet black, and snowy white (often with brown, green and yellow plumage, too), and the bird creates unmistakable, loud mallet-like hammering and pounding noises by repeatedly pecking on standing and most often hollow trees in search of insects. Pounding on a long, hollow wooden tube will create a huge ruckus, heard a long distance in a quiet forest, and the bright plumage is easily spotted on a standing dead tree. The shape of the right element itself is similar to a merging of「bipedal」(birds and humans walk upright on two legs without using a tail) and a pictograph of ribs and spine, also meaning VERTEBRAE, as is a standing dead SHII tree with only remnants of broken branches protruding horizontally from the trunk, again resembling a SPINE. The WOODPECKER uses its entire SPINE to forcefully peck like a MALLET into SHII trees.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: spine; a hammer
Japanese EDRDG: oak, mallet
Unihan extended: hammer, mallet; vertebra

Used within names (nanori)
しい