Mouth movement or mouthing is part of non-manual grammar in sign language. When a mouth morpheme is used, it conveys an adjective, adverb, or another descriptive meaning in association with an ASL word.
Some ASL signs have a permanent mouth morpheme as part of their production. For example, the ASL word NOT-YET requires a mouth morpheme (TH) whereas LATE has no mouth morpheme. These two are the same signs but with a different non-manual signal.
These mouth morphemes are used in some contexts with some ASL signs, not all of them.
MM: moderately, normally, average, medium; with ease.
OO: small, tiny, lightly, slowly. It may mean otherwise in some contexts (e.g. very long in time as in Y-LONG-head).
CHA: very big, very large, gigantic, very tall.
AAH: far. It can be another meaning in some contexts (e.g. LONG-WAIT-cont).
CS: very close; with effort or struggle.
TH: carelessly, foolishly, distractedly; melting. E.g.: melting (eg. snow, ice, chocolate), nothing (in some contexts), sticky as in glue (sometimes), sloppy, careless (in some contexts).
PUFF-cheek: fluffy, bubbly, and few other meanings. Seen in HOLD-BACK-LAUGHTER, SHORT-AND-SWEET-CUTE, etc.
Puckered lips: leisurely, deliberately. Seen in STROLL or AMBLE.
Below shows some examples of mouthing that represents a size of a book. Facial grammar is integrated with a classifier (a handshape that represents a group of objects).
A number of the pages of a book is average.
A book is thick in pages. Notice a different handshape from the previous one.
The length of pages of a book is short.
It means finally! or success at last!, that one finally or successfully does something after a long while of a struggle, hard work, or long effort.
MA (if not PAH): tend-to.
Also see non-manual signals in sign language.