What does the Thai word ‘nong’ mean?

“Nong” (น้อง) means a younger sibling. We can also use it to refer (informally) to someone younger than us.

Examples:

If your younger sister is named Ploy, you can call her “Nong Ploy”
If you meet a friend who’s younger than you named Ton, you can call him “Nong Ton”
If you want to refer to someone who’s younger than you, but you don’t know his/her name, you can simply call him/her “Nong”