Wood from the tree ochroma lagopus. Light wood that easily foats and is breakable. Wood used for modelling. A piece of balsa: A useless item liable to break. Not well constructed. cf "pants, a pile of"
"That car's a piece of balsa" and "You can make a model out of that!" (ie. it's balsa) as fearured on an Alexei Sayle recording: "Hello John Got a New Motor?"