To say Uncle in Thai you have to specify whether it’s your father’s or mother’s older or younger brother.

Uncle from Father’s side

If it’s your father’s younger brother then uncle is อา (aa).

If it’s your father’s older brother then uncle in Thai is ลุง (lung).

Uncle from Mother’s side

If it’s your mother’s younger brother then uncle is น้า (náa).

If it’s your mother’s older brother then uncle in Thai is ลุง (lung).

Learn the words for aunt here.

Example

My older uncle is older than my younger uncle by 2 years. As for my dad, he’s right in between them.
ลุงของฉันอายุมากกว่าอาสองปี ส่วนพ่อฉันอยู่ตรงกลางระหว่างสองคน
lung kŏng chăn aa-yú mâak gwàa aa-sŏng-bpee sùan pôr chăn yòo dtrong glaang rá-wàang sŏng-kon

Summary

To summarize, uncle in Thai depends on whether it’s your uncle from the father’s or mother’s side and also whether it’s their older or younger brother.

In both sides, the older brother is ลุง (lung).
In your father’s side if its the younger brother then uncle in Thai is อา (aa).
In your mother’s side if its the younger brother then uncle is น้า (náa).