Actual Gesture: Covering the mouth with the tips the fingers, whether it's the front, back in a cup shape
It's Function?: To hide your teeth and cover up while you hold back the chuckles
Any Associated Phrase?: No, but the implied one is “Sorry I find your current state hilarious”
Anything else?: It's a rather formal-and perhaps feminine?-gesture, and back in the day, flashing the choppers meant either showing the person in front of you a crack in your armor or that you weren't that serious of a person, either implying poor form on your part; in addition to that, women had to deal with the fact that pearly whites meant you weren't hitched yet-and possibly never would be, since married women had theirs painted black, the practice known as 御歯黒「おはぐろ」. These days, though, it's more likely they think they got a bad case of the yuck-mouth, and would prefer you not see them on display.
Phrase #6:
Literal Meaning: There is business
Intent: I got a thing do to
Usage: Much like the phrase "I got a thing to do", it tells that there's something else that needs our attention without specifying what it is, thus insuring that your private and public lives stay separate.
Notes: This phrase also has few interesting variants to it, like 用がない(it and any variant of it saying you either don't have anything to do or that you have no need/use for something) and 用を
Example: (用がある)「用がある」って?
(So you 'got something to do,' eh? I get you, man. See you 'round)
(用がない) 用がなかったら、
(If it's not useful to you, why not just throw it out?)
(用を足す)
(Pardon the interruption. Off to go take care of business)