Variation of the traditional "settle down". Settle-D is a street smart phrase used mostly in an urban context when someone overreacts or simply reacts inappropriately to a situation/event. Settle-D is most relevant when the overreacting person is demonstrating too much excitement for their own good and is placing the social decorum of the party in danger.
Overreacting friend: "OMG dude! Look at the chicks that just walked in! I am totally going to impress them by buying a round of shots and showing them my sick dance moves!"
Other friend: "Settle-D bro"