To the definition above. NO! HITCHHIKING IS NOT A GAY SEX ACT YOU RETARD!
Anyway, hitchhiking is the method of transport/lifestyle in which a person or group of people stand by a major motorway and wait for a free ride. It is risky but the likelihood of being raped and chopped into tiny bits is very small and overplayed by the fear-mongering media. In my opinion.

when doing a girl doggstyle, the gentleman inserts his thumb in said girl's butthole

when a dude is thinking of another dude while sticking his thumb up his ass.

The action of thumbing a person either in their anal cavity, or the vaginal cavity.