In USA and UK we take the "thumbs up" sign to be an indication of approval. It is used in many different scenarios including the "thumbs up" made famous by the Stumbleupon social network site.
The thumbs up sign was originally used by the ancient Romans to decide the fate of Christians in the arena. The thumbs up (pollice verso) sign meant "kill him" and the thumbs down, or hidden (pollice compresso) sign meant "spare him". In the film "Spartacus" the directors actually got this the wrong way round.
In most of the world it's quite acceptable to offer the thumbs up to people but if you are traveling in Afghanistan, Iran, Nigeria, or parts of Italy and Greece, take care. In these countries it is considered extremely rude and insulting. In Sardinia it is seen as particularly offensive and a hitch hiker thumbing a lift is more likely to be run down, than offered a ride.
Roughly speaking, in these countries, this hand signal is the equivalent to the "middle finger" salute which is common in USA and UK.
The term "thumbs up" is now used in English speech. For example, "The director gave me the thumbs up." In formal circumstances that phrase is acceptable where actually giving the thumbs up sign might not be.
So, it pays to research the customs of any country you might be visiting before you use the "thumbs up" or any other type of hand signal. It just keep you out of trouble, or at least improve your chances of thumbing a lift in Sardinia.
