Middle English (in an earlier sense). From over + -more. Compare Old Icelandic ofar meir, Old Swedish ivirmer, Old Danish owermerae (Danish †overmere).
Definition of overmore in US English:
overmore
adverbˌəʊvəˈmɔː
rare
In addition; furthermore, moreover.
Origin
Middle English (in an earlier sense). From over + -more. Compare Old Icelandic ofar meir, Old Swedish ivirmer, Old Danish owermerae (Danish † overmere).