phrasal
1. : to say nothing of : not to mention
but does he have the boldness, let alone the skill, of his own convictions — New Republic
no provision for the health of servants, let alone for their comfort — O.S.J.Gogarty
2.
a. : to refrain from interfering with : leave undisturbed
in the spring or summer, let nests alone — Boy Scout Handbook
the government … promised to let them alone in part of the Everglades — Marjory S. Douglas
could not let well enough alone — L.O.Coxe
— often used as an imperative
let it alone; you'll break it
b. : to leave to oneself
the red-faced boy wanted nothing so much as to be let alone
her mother told her to let the frightened kitten alone
3. : to exclude from consideration
the … party is inclined to let nationalization pretty well alone — Alzada Comstock