Where does “cmbk'” come from?

cmbk' (Middle Persian) comes from Sanskrit चम्पक, from Pali campaka.

Ancestry of “cmbk'”, step by step

Step	Language	Word	Meaning
1	Sanskrit	चम्पक	a tree bearing yellow, fragrant flowers (Magnolia champaca, formerly Michelia campaka); the champac tree
2	Pali	campaka	—