Do you mean what is Thank you in Tagalog or do you mean what is the etymology of the equivalent of Thank you in tagalog?
Thank you in tagalog is salamat. For sayng to someone who is of more stature or for rendering respect, we say"salamat po".
Etymology: Philippines was predominantly Muslim before the Spaniards came to colonize the land, so the word came from the words Asa allah ma ak. Meaning may god be with you for helping me. But due to the discourse of time, it became to be Salama'at and became Salamat.