Douglas Harper's Etymology Dictionary
1610s, from un- (1) "not" + preventable (adj.).
Wiktionary
a. Not preventable.
WordNet
adj. not preventable; "unpreventable hysteria" [ant: preventable]
Usage examples of "unpreventable".
The unexplainable and unpreventable were part of life, and people learned to live with them.
Human was coming off the bench with fire in his eyes, undeterred by extraneous and unpreventable problems.
It was as predictable and as unpreventable as the tides or the phases of the moon.
An ex-Imperial assassin who knew, bone deep, that he could bring an unpreventable death to everyone around him.
Her long, low moan was music in his ears, although his own unpreventable deeper groan smothered it.
For Kraus, prostitution is a natural and unpreventable practice, and attempts to impose legal control are inherently hypocritical.
When the unpreventable collapse occurred, in Vietnam and in Cambodia, in April and May 1975, the cost was infinitely higher than it would have been seven years previously.