Yat is a Thai term which means "to stuff". Yat sai, on the other hand, means "stuffed", as in Thai dishes like Stuffed bell peppers called Prik yat sai in Thai
Yen is a Thai word meaning cold.
Yen also refers to early evening, between 4:00 and 6:00 in the afternoon, as in "kao yen" for evening rice or "dinner".
Yen literally means "early"