Stuffed-up describes the feeling when a cold has blocked your nose, so air will not pass through properly and you have to breathe through your mouth. It also affects how you sound — a stuffed-up voice is that flat, thick voice everyone recognises as 'she has a cold'.

You can use it about the person or about the nose: 'I'm all stuffed-up today' and 'my nose is stuffed-up' both work. It is informal, everyday spoken English, exactly what you say to a friend or a colleague. The medical word for the same thing is 'congested', which is what a doctor writes down; nobody says 'I am congested' at breakfast.

The picture behind it is the ordinary verb stuff — to pack something full. Your nose feels as though someone has packed it with cotton. That image is why the word feels so natural to native speakers even though nothing is really inside.

American speakers often shorten it to 'stuffy' for the same idea — 'a stuffy nose'. Be careful, because 'stuffy' has other meanings too: a stuffy room is one with no fresh air, and a stuffy person is boringly formal. Only with 'nose' does stuffy mean blocked.

Writing note: the hyphen is common but not fixed. You will see 'stuffed up' without it, especially after the verb: 'My nose is stuffed up.'