Regarding the noun "witness" (`edh), the Theological Wordbook of the Old Testament says: "A witness is one, who by reiteration, emphatically affirms his testimony."
A Comprehensive Etymological Dictionary of the Hebrew Language for Readers of English adds: "The orig[inal] meaning [of the verb `udh] prob[ably] was `he said repeatedly and forcefully.'"
According to the Theological Dictionary of the New Testament, "the concept of witness [is used] both in the sense of witness to ascertainable facts and also in that of witness to truths, i.e., the making known and confessing of convictions." So a witness relates facts from direct personal knowledge, or he proclaims views or truths of which he is convinced.
According to the Bible, the line of Witnesses of Jehovah reaches back to faithful Abel. Hebrews 11:4-12:1 mentions this line as a "great a cloud of WITNESSES surrounding us."
The Bible says that Jesus Christ was the foremost witness of Jehovah: "These are the things that the Amen says, the faithful and true witness, the beginning of the creation by God." (Rev. 3:14) Considering the definition of "witness" mentioned above, Jesus said that it was his Father's name that he made manifest. (John 17:6)
The beliefs and practices of modern-day Jehovah's Witnesses are not new but are a restoration of first-century Christianity.
"Ye are my witnesses, saith Jehovah, and my servant whom I have chosen;" (Isa. 43:10) -ASV