An Open Mouth is Impure:

Sefer Avodas Yisroel (Parshas Para) explains this verse to be referring to the importance of guarding one's words. It can be understood as saying that if one's mouth is always open, meaning that he says whatever pops into his head, he will inevitably end up speaking lashon hara and profanity. Thus, if one's mouth is an "open vessel", in the sense that one has "no seal”, i.e., no filter on his words, he is impure, as he will certainly engage in forbidden speech.