The property of hydrogen that made it less than desirable as the fill-gas for large dirigibles is its extreme flammability.

Here's why:

* Highly flammable: Hydrogen is the most flammable element, meaning it ignites easily and burns rapidly.

* Low ignition energy: Hydrogen requires very little energy to ignite, making it susceptible to accidental fires.

* Rapid combustion: Hydrogen burns with an intense heat and produces a large amount of energy, leading to devastating explosions.

This flammability was tragically demonstrated in the Hindenburg disaster in 1937, where a hydrogen-filled airship caught fire and exploded, killing 36 people.

Although hydrogen offered the advantage of being lighter than air, the risk of fire was simply too high, leading to the switch to the less flammable helium for airships.