To smack someone on the back of the head with an open hand. Derived from the trademark slap delivered by the character Leroy Jethro Gibbs to his underlings on the T.V. show N.C.I.S. To deserve a 'gibbs' or a 'gibbs-smack' the person getting it should have done something stupid, vain or dangerous.
When I found out he was cheating on his gf that was one thing; when I found out the asshat was hitting on jailbait in her favorite bar, I gave him a gibbs he'll never forget.