Learn English – Shut and close your mouth

Neither the Oxford English Dictionary, ed. 1 nor the English Dialect Dictionary records this precise use, but Puck and Simon clearly use lie here in a sense similar to that found in "lie in one's power": that is, to lie in your mouth is roughly equivalent to "be proper for you to say".

Puck questions Simon's competence or authority to contradict Puck's characterization of Drake, and Simon responds "Why shouldn't I be competent, since I was the first to know him?"

The uninflected do is dialect.

The shape of the closed mouth can be referred to as a line. When you are smiling the line of the mouth curves upwards, from the middle to both ends. When you are frowning the line of the mouth curves downwards, from the middle to both ends.

When someone has a grim face, their mouth is more likely to be relatively straight, not curving in either direction, with the lips curving slightly in on themselves and pressed firmly together. The eyes are usually focused directly in front and tend not to move around as much as normal, in fact the face as a whole shows little animation. This is caused by the facial muscles, and the small muscles around the eyes, being stiff and tense.

A grim face is usually a sign that someone has experienced something extremely upsetting and they are trying to keep themselves composed; or they have an unpleasant and difficult task to perform, which probably has a low chance of success, but the are determined to overcome the obstacles that they are facing and eventually succeed. Obviously, in this case, we are talking about the first situation.