Rihanna -- Sued for Defamation by Former Bodyguard

Rihanna is known to talk her fair share of smack, but a former bodyguard claims she went over the line and is now suing the singer for defamation ... TMZ has learned.

Geoffrey Keating filed a lawsuit today in Ireland, where his attorney was also granted a motion to serve Rihanna with the suit in the U.S.

Keating claims Rihanna made "nauseatingly offensive" and false statements about him in an email to him and his partner last July. He also claims the offensive statements were made in a phone call to his sister.

Keating served as head of security for Rihanna while she was on tour between October 2012 and July 2013. He claims the remarks have gravely injured his reputation and his business.

Keating's attorney, Gerald Kean, tells TMZ, "We're taking this battle very seriously. We are satisfied that the Irish Courts will see justice at the end of the day."