It's common knowledge that the sign for hitchhiking is to stick your thumb out. But how did it come to be that? Why not the index finger or simply just hold the entire hand out, fanning it vigorously until the car stops? At Today I Found Out, they explored various angles as to the explanation why the "thumbs up" sign became the universal symbol for hitchhiking and the history of the sign itself.
The most convenient and perhaps reasonable explanation is that it originated from the Flying Tiger brigade of American pilots who picked up the gesture from the Chinese when they were stationed there. After arriving back in the US, it simply caught on, first in the military and then to ordinary civilians. However, that meaning of the "thumbs up" sign only came about after WWII. The odd thing is that records have shown that the terms hitchhiking and "thumb-pointer" (to refer to a hitchhiker) had already been used before WWII.
But generally, it appears that the reason why hitchhikers would stick out their thumb was simply to indicate the direction in which they were going. Considering how awkward the gesture would be if you used any of your other fingers, the thumb sticking out is the best way to go.
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