A Doriathrin noun meaning “the exterior, the outside”, literally “outside the fence” (Ety/GAT(H)). It is a combination of ar- “outside” and gad “fence”. The entry in The Etymologies also had a rejected variant argadon (EtyAC/GAT(H)).
argadnoun. ‘outside the fence’, the exterior, the outside, exterior, outside, (lit.) outside the fence