early 15c., "of or pertaining to testimony," in part from testimonial (n.) and from Late Latin testimonialis, from Latin testimonium (see testimony). Originally especially in phrase letters testimonial (French lettres testimoniaulx, Latin litteræ testimoniales) "document or documents attesting to a fact or to the good standing of the bearer," literally "letters serving for evidence."
"statement, declaration," also "writing testifying to one's qualification or character," early 15c. (from Old French testimonial, variant of tesmoignal), short for letters testimonial (see testimonial (adj.)). Meaning "gift presented as an expression of appreciation" is from 1838.