You are not able or willing to talk because of shyness. Usually used about children or as a question to children. * /Tommy's father asked Tommy if the cat had got his tongue./ * /The little girl had a poem to recite, but the cat got her tongue./ Compare: LOSE ONE'S TONGUE.
CAT GOT ONE'S TONGUE
Meaning of CAT GOT ONE'S TONGUE in English
Slang English vocab. Английский сленговый словарь. 2012