Kevin's friend George is getting married to Sharon. A week before the wedding, George invites his friends to a stag party. They start in a pub, and then go on to a club where there is deafening music and they all drink too much.
The telephone rings. Joanne, Kevin's girlfriend, answers it. It is Kevin's boss, who wants to know why Kevin is not at work. “He's feeling a bit UNDER THE WEATHER”, explains Joanne. “He'll be in tomorrow”.
The expression “under the weather” means “not very well” – not seriously ill, just not very well. If I have a slight cold, or I am feeling a bit depressed, for example, I might say that I am “under the weather”.
To understate something is to say less about it than you really mean. For example, you might say that someone is “a bit annoyed”, when in reality they are very angry. Or on the hottest day of the summer, you might say that it is “a little bit warm”.
To return to Kevin. He spends the day with his eyes closed complaining about how awful he feels. Joanne is unsympathetic. “It's his own fault for drinking too much”, she says. That evening, Joanne gets ready to go out. “Where are you going?” asks Kevin weakly. “To Sharon's hen party. Go back to bed”.
A vocabulary note. A stag party, or a stag night, is a party given by a man about to get married for his male friends. A hen party is given by a woman about to get married for her girl friends. The guests at these parties often drink too much and do outrageous things. I never go to them, of course.