formal : to show, prove, or state that something is true or real I can attest that what he has said is true. The certificate attests the authenticity of the painting. He was asked to attest [=authenticate] the will/signature.

What does it mean to attest a will?

Definition. Testimony or confirmation that something is true, genuine, or authentic. An attestation is frequently in writing. For example, a witness attests a will by signing it; his or her signature may confirm, inter alia, that he or she witnessed the testator sign the will.

What does it mean to attest a document?

Attestation is the act of witnessing the signing of a formal document and then also signing it to verify that it was properly signed by those bound by its contents. Attestation is a legal acknowledgment of the authenticity of a document and a verification that proper processes were followed.