1803, "pertaining to or of the nature of a phenomenon," a hybrid from phenomenon + -al (1). Meaning "remarkable, exceptional, exciting wonder" is by 1850.
[Phenomenal] is a metaphysical term with a use of its own. To divert it from this proper use to a job for which it is not needed, by making it do duty for remarkable, extraordinary, or prodigious, is a sin against the English language. [Fowler]