Definition of remainer:
(n) :
One who remains.
(n) :
(UK politics) Someone who supports the United Kingdom remaining within the European Union.
(n) :
(UK politics) Alternative letter-case form of Remainer. [(UK politics) Someone who supports the United Kingdom remaining within the European Union.]
(n) :
Obsolete form of remainder. [A part or parts remaining after some has/have been removed or already occurred.]