Hearing the word trustable used twice on National Public Radio (NPR) in what seemed to be a serious context, I decided to explore the usage.
Although the form trustable can be documented—the OED cites examples dated 1606, 1884, and 1900—it’s a rarity in modern usage.
Both, trustable and untrustable make a slight showing on the Ngram Viewer beginning in 1824 and 1852, respectively, but compared to trustworthy and untrustworthy, they never rise from the very bottom of the graph.
A Google search brings up about 627,000 results for trustable, compared to about 60,900,000 for trustworthy.
A search for untrustable leads inexorably to this statement by Kevin McCarthy:
What are her numbers today? Her numbers are dropping. Why? Because she’s un-trustable.
Like David A. Graham, I was ready to attribute the suddenly popular usage of untrustable to McCarthy:
Color me unimpressed. The most surprising thing about this quotation is McCarthy’s coining of the word “untrustable.”—“Kevin McCarthy Steps Into a Faux Outrage,” The Atlantic, September 30, 2015.
The credit/blame for introducing the form untrustable can be traced further back than McCarthy. The OED has a single citation, from 1863: “Dennis will look up at you with his…good-natured, untrustable, Irish grey eye.”—Charles Kingsley.
In 1997, the group Built to Spill released an album called Perfect from Now On. One of the songs is called “Untrustable/Part 2.”
The forms trustable and untrustable are not barbaric in the way so many recent coinages are, but they’re not necessary. And to many speakers, they sound like errors for trustworthy and its negative form, untrustworthy.
trustworthy adjective: Worthy of trust or confidence; reliable.