The story
Contraction of let us. Compare non-contracted Dutch laat ons, German lass uns (speaker with one other person) / lasst uns (speaker with more other persons) and Swedish låt oss.
Etymology from Wiktionary, CC BY-SA 4.0.
let's
Today it means: Used to form the cohortative of verbs, equivalent of the first-person plural imperative in some other languages. Full definition →