From Longman Dictionary of Contemporary Englishslap somebody on the backslap somebody on the backto hit someone on the back in a friendly way, often as a way of praising them → slap
Examples from the Corpus
slap somebody on the back• Forbes slapped me on the back.• In those days it was considered bad form even to slap you on the back.• She slapped him on the back.• He rushed over and slapped me on the back as I lifted a highball to my lips.• She intended to slap Elisabeth on the back, but she was too late.• I go to slap her on the back but she waves my arm away impatiently.• Jack said, and he slapped Streeter on the back of the head with his gun hand. 