in which country do people greet each other by locking thumbs?

In Zambia, people often greet one another by locking thumbs, which is equivalent to shaking hands in the United States.

Locking thumbs may be a sign that you’re about to lock arms and dance with someone! Among many different tribes in Zambia, locking of thumbs between two persons denotes a very close bond. It is a sign of friendship between couples or mutual respect and acknowledgement within the extended family. Other hand motions such as slapping palms together and rubbing an earlobe are also used as complimentary gestures when meeting one another for the first time or exchanging recognition of past acquaintanceship.