Early 19th century; earliest use found in Noah Webster (1758–1843), lexicographer. From ancient Greek ὄνομα name + -mantic, with haplology, after onomantical.
Definition of onomantic in US English:
onomantic
adjectiveˌɒnəˈmantɪk
1Of or relating to onomancy.
2Onomastic.
Origin
Early 19th century; earliest use found in Noah Webster (1758–1843), lexicographer. From ancient Greek ὄνομα name + -mantic, with haplology, after onomantical.