¡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)

1st Person Plural	2nd Person Plural
levantemos	levantad

levantarse (imperativo)

1st Person Plural	2nd Person Plural
levantémonos	levantaos

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).