The word means that with which the anointing is performed - the unguent or ointment. In the New Testament only here and 1Jo 2:27. Rev., an anointing. The root of this word and of Χριστός, Christ, is the same. See on Mat 1:1. The anointing is from the Anointed.
The best texts read πάντες, ye all know; in which case the connection is with the following clause: “I have not written unto you because ye know not the truth, but because ye know it.”