Mid 19th century; earliest use found in Charles Kingsley (1819–1875), novelist, Church of England clergyman, and controversialist. From unrideable + -ly.
Definition of unrideably in US English:
unrideably
(also unridably)
adverb(ˌ)ʌnˈrʌɪdəbli
So as not to be rideable.
Origin
Mid 19th century; earliest use found in Charles Kingsley (1819–1875), novelist, Church of England clergyman, and controversialist. From unrideable + -ly.