Thing Not To Be Today: thrasonical.

 The British don't much like people who boast. If one of us is given an opportunity to boast (which won't happen very often) then the correct thing to do is to reject the opportunity with self-deprecating humour.

Many societies in the world do not understand this behaviour at all. In fact, I'm not even sure that I do.

(The British also don't much like people who expect to be the centre of attention, either, and it's possible the two things are linked.)

Still, if you want to describe someone as boastful without making an enemy then you can always use the word thrasonical. 

Even if the person in question is listening - and boastful people often don't listen - then it's very unlikely he or she will understand it.

Thing Not To Be Today: thrasonical. Thrasō is a boastful soldier in a dodgy but very-successful-at-the-time play, Eunuchus, by the Roman writer Terence. 

woodcut by Albrecht Dürer 

The Greek word thrasus means forceful.