of or relating to the Arabian prophet Muhammad or to the religion he founded

Mohammedan (variant form: also Muhammadan) is a term used as both a noun and an adjective meaning belonging or relating to either the religion of Islam or to that of its last prophet Muhammad. The term is now largely superseded by Muslim, Moslem or Islamic but was commonly used in Western literature until at least the mid 1960s. (See for instance the second edition of "A Dictionary of Modern English Usage" by HW Fowler, revised by Ernest Gowers (Oxford, 1965)).