The root word of bitachon is betach/certain, for bitachon isn’t only a belief in G-d’s ability, but a sense of certainty that everything will be good in a revealed manner, until it envelops one with tranquility.
Within bitachon is the word tach/plaster, since plaster is a connecting agent, holding together all that it covers, like a friendship bringing people together and forging a natural connection. Good friends naturally trust each other, greater the friendship, greater the bond, greater certainty that each will only do what is in the other’s best interest.
Our bond with G-d so strong, we’re plastered together. ~ Lubavitcher Rebbe