To let a frog out of my mouth:
to say wrong thing,
It’s a translated idiom of Finnish Language which mean “to say something wrong by mistake which embarrass or upset someone”.
It’s similar to English Idiom ‘Put your Foot in it’
Sam: Eh! I really let a frog out of my mouth I didn’t know that she was from village.
In the example Sam talks bad about rural people to a girl who herself was from a village.