means a document or token that confirms a payment, entitlement, or claim — exchangeable for goods, services, or money, or serving as proof that something is true.

from From the verb "vouch," meaning to guarantee or bear witness, which came through Old French "voucher" (to summon, call upon) from Latin "vocare," to call — itself rooted in "vox," voice. So a voucher is literally a thing that speaks up on your behalf: paper raising its hand to swear you're good for it. The "-er" ending turned the act of vouching into the object that does the vouching.