على راسي

عندما تقول بأن شخصًا على راسك معناهُ أنك تكن لهُ الاحترام.

on my head, my first priority
can be said to reassure someone or say okay, or to thank someone, say your welcome etc

It literally translates to "on my head".

It's basically a way of saying you respect something/someone. It is used in many contexts but this is the most standard one.

Another one is if someone does a favor for you or even if you're paying someone to do a job and you thank them for it, they might say this.