Webster's Revised Unabridged Dictionary · 1913

Testify

Testify , intransitive verb

[Old French testifier, Latin testificari; testis a witness + -ficare (in comp.) to make. See -fy, and compare Attest, Contest, Detest, Protest, Testament.]

1.
To make a solemn declaration, verbal or written, to establish some fact; to give testimony for the purpose of communicating to others a knowledge of something not known to them.
Jesus... needed not that any should testify of man, for he knew what was in man. — John ii. 25
2.
(Law) To make a solemn declaration under oath or affirmation, for the purpose of establishing, or making proof of, some fact to a court; to give testimony in a cause depending before a tribunal.
One witness shall not testify against any person to cause him to die. — Num. xxxv. 30
3.
To declare a charge; to protest; to give information; to bear witness; -- with against.
O Israel,... I will testify against thee. — Bible (KJV) - Psalm l. 7
I testified against them in the day wherein they sold victuals. — Neh. xiii. 15

Testify , transitive verb

1.
To bear witness to; to support the truth of by testimony; to affirm or declare solemnly.
We speak that we do know, and testify that we have seen; and ye receive not our witness. — John iii. 11
2.
(Law) To affirm or declare under oath or affirmation before a tribunal, in order to prove some fact.

Testify , adverb

In a testy manner; fretfully; peevishly; with petulance.