expression, godan verb (-ku)
speak reluctantly; open one's heavy mouth
Used when someone finally speaks after hesitation, often about a difficult or unpleasant topic. The image is of a mouth that feels heavy and hard to open.
彼はしばらく黙っていたが、ようやく重い口を開いた。
He was silent for a while, but finally opened his heavy mouth and spoke.
母はなかなか重い口
My mother didn't open her heavy mouth for a long time, looking like she wanted to say something.