Allow to escape, set free, as in The police decided to let him go. [c. 1300]
Also, . Release one's hold on, as in Please let go of my sleeve, or Once he starts on this subject, he never lets go. [Early 1400s]
let it go. Allow it to stand or be accepted. For example, Let it go; we needn't discuss it further. This usage is sometimes amplified to let it go at that, meaning "allow matters to stand as they are." [Late 1800s]
Cease to employ, dismiss, as in They had to let 20 workers go.
Also, let oneself go. Behave without restraint, abandon one's inhibitions; also, neglect one's personal hygiene and appearance. For example, When the music began, Jean let herself go and started a wild dance, or After her husband's death she let herself go, forgetting to bathe and staying in her nightgown all day. The first sense dates from the late 1800s, the second from the early 1900s.