Hello Alice, You've already known in our previous class that, "Bermuka tebal" is an idiom to say that someone is cheeky. Or people more likely to say "orang ngga tahu malu" rather than to say this idiom. There is another idiom such "tangan panjang" means that he/she likes to steal something.

It means "gak tahu malu" as I understand it.

bermuka tebal is shameless.