RMB noun3 •Initialism of right mouse button. •(US, Australia) Initialism of roadside mailbox: part of a postal system used in rural areas where several houses collect their mail from a cluster of mailboxes at the roadside.Address: RMB 35 Beaumont Crescent, The Ridgeway, Queanbeyan. New South Wales 2620, Australia. — 1997 •(UK) Initialism of Royal Marine Base. formsRMBs