Definition of "unstable"

The hillside at this point is composed of shaly rock overlaid with a peaty loam which carries a growth of heather, and its unstable condition has resulted in two landslides in the course of the railway's history.

1943 March and April, “A British Avalanche Shelter”, in Railway Magazine, page 80

The dawn of the oil age was fairly recent. Although the stuff was used to waterproof boats in the Middle East 6,000 years ago, extracting it in earnest began only in 1859 after an oil strike in Pennsylvania. […] It was used to make kerosene, the main fuel for artificial lighting after overfishing led to a shortage of whale blubber. Other liquids produced in the refining process, too unstable or smoky for lamplight, were burned or dumped.

2013 August 3, “Yesterday’s fuel”, in The Economist, volume 408, number 8847

When the last tune of music floated from the fleet, he unstabled his quarter horse and headed for the coastal road leading west and north on his circuit.

1992, Elizabeth Darracott Wheeler, Sir John Dodderidge, Celebrated Barrister of Britain, 1555-1628