Combat Dildo

A flexible foam rubber stick (available in 'sky blue' or 'sunset yellow'), which can undergo a chemical change in its elastic layers to become rigid and straight in seconds. Often used to train people in sword fighting, because a blow to the head makes the 'blade' bend rather than causing injury; but can still knock the loser down safely as it solidifies.

The trade name for these devices is Combat DLD (for Dynamic Laminar Device), but it seems military trainees find the acronym takes too long to say.