a slap on the back
an action of hitting someone on the back in a friendly way, especially as a way of praising them
slap somebody on the back
to hit someone on the back in a friendly way, often as a way of praising them

[TahlilGaran] Dictionary of Contemporary English ▲

a slap on the back
praise or approval.
We gave her a big slap on the back for helping to organize the concert.

[TahlilGaran] English Idioms Dictionary ▲