hat S1 W3 [countable]
a piece of clothing that you wear on your head:
Maria was wearing a beautiful new hat.
in a hat
a man in a fur hat
bowler-hatted/top-hatted etc (=wearing a bowler hat, top hat etc)
a bowler-hatted gentleman
to keep something secret
3 also have your teacher's/salesman's etc hat on informal
to be performing the duties of a teacher etc, which are not your only duties:
I'm a manager now and only put my salesman's hat on when one of our sales reps is having real problems.
4 also hats off to somebody informal
used to say you admire someone very much because of what they have done:
I take my hat off to Ian - without him we'd have never finished this project on time.
if someone's name is drawn out of a hat, they are chosen, for example as the winner of a competition, because their name is the first one that is taken out of a container containing the names of all the people involved:
The first correct entry out of the hat on September 2nd will win a prize.
to collect money from a group of people, especially in order to buy someone a present