Shaking hands has always been a symbol of trust and agreement in business deals for a very long time. There are several synonyms for the phrase "shake on it" that can be used for informal and formal agreements such as agreeing on a bet. "Seal the deal" is a term that is often used to emphasize the completeness of the agreement. "Make a pact" is another synonym for the same phrase that is used to signify a solemn and binding agreement. In informal settings, "let's pinkie swear" or "let's high-five" can be used to make an agreement more playful. Ultimately, whichever term is used, it all is about building trust in a professional or personal relationship between two individuals.