Wood is a porous and fibrous structural tissue found in the stems and roots of trees, and other woody plants.
It is well known in the Nedverse for being Ned's allergy. Ned and other people in his family suffer from sensitivity to wood and the allergy is so strong that it can be fatal at times.
This is why Ned always asks "Do you haaave any wood?! Hmmm?!" as a precaution.