D defs.my
Entry 1 sense Webster, 1913

Unless

/ən-lĕs'/ · Un·less · IPA /ɪnˈlɛs/
01 conj. Upon any less condition than (the fact or thing stated in the sentence or clause which follows); if not; supposing that not; if it be not; …
  1. 1.
    Upon any less condition than (the fact or thing stated in the sentence or clause which follows); if not; supposing that not; if it be not; were it not that; except; as, we shall fail unless we are industrious.
    “Here nothing breeds unless the nightly owl.” Shak.