EXCLUSIVE

"Big Brother" star Cody Nickson didn't just serve his country as a Marine for 4 years, he was also twice deployed and received numerous medals ... according to his file obtained by TMZ.

Cody's been on the outs with the rest of the cast except his gf, Jessica, but things got nasty this week when some of them -- including Paul Abrahamian -- tried antagonizing the Marine Corps vet by questioning his military experience ... and 'BB' fans were pissed.

Rightfully so, it turns out, because according to Cody's official military file ... he served in the Marine Corps from December 2008 to December 2012, reaching the rank of E-4 Corporal with a specialty as a Rifleman. He previously served in the U.S. Air Force as well.

Nickson received numerous honors ... including campaign medals for his tours in Afghanistan and Iraq, 2 National Defense Service Medals, a Good Conduct Medal, a Global War on Terrorism Service Medal and 2 Navy Unit Commendations.