I. phrasal
: to leave undisturbed
let the flowers alone
also : to leave to oneself
wanted to be let alone
II. conjunction
Date: 1765
: to say nothing of : not to mention — used especially to emphasize the improbability of a contrasting example
he would never walk again let alone play golf — Sports Illus.
how many ever see an Ambassador or Minister, let alone a President — Robert Lacville