underarch

under- +‎ arch

underarch (third-person singular simple present underarches, present participle underarching, simple past and past participle underarched)

1. (archaic, poetic, transitive) To form an arch beneath.
   • 1886, Samuel Rutherford Crockett, Dulce Cor: Being the Poems, page 45:

     And the trees that underarch the sky / And shadow the dreaming river.