slap (one) on the back
To give one a sign or expression of praise or approval. Brett's boss slapped us all on the back after we completed the project early and under budget.
slap on the back
An expression of approval or praise. Brett's boss gave him a slap on the back after he completed the project early and under budget.
slap on the back
A gesture of congratulation, as in The coach gave him a slap on the back for coming in first. [Early 1800s]
slap someone on the back
congratulate someone heartily.