¡Larguémonos!

¡Larguémonos!
 

Hi,

¡Larguémonos (de aquí)! means "let's get out of here!", clear enough. It looks like larguemos + nos to me (Present Subjunctive of largar). Is there a rule about dropping s from emos in such a case? It looks quite natural, yet I can't find it.

When suffixing the first-person plural reflexive pronoun to the imperative, the 's' is dropped from the first-person plural ending.
A similar rule exists for the second-person imperative of a pronominal verb, only it is the 'd' that is dropped.
This is explained here, for example.

levantar (imperativo)

levantarse (imperativo)

Vamonos is a common example

I agree that vámonos is commonly used, but this particular verb doesn't adhere to the normal rules (check out this post for more information).