Don't be mamus is a derivation of the Spanish Expression "No Mames" or the softer version "No Manches" This expression is widely used in Oaxaca de Juárez, Mexico to indicate that someone is taking advantage of you in some way. It can also portray the sense that someone is exagerating some point. It also can be used to mean "really?" or "you don't say" Additionally it has been used to express an informal greeeting such as "hey" or "what's up?" or in departing a "goodbye".
Someone taking advantage of you. "They charged me fifty dollars for the repairs" Response: "Don't be mamus"
Greeting...said as one approaches friends: "Don't be mamus"
Goodbye...as one leaves the presence of friends "Don't be mamus"
Surprise..."I won the lottery" Response "Don't be mamus!"