neuter of a presumed derivative of 3144; something evidential, i.e. (genitive case) evidence given or (specifically) the Decalogue (in the sacred Tabernacle):

KJV-- to be testified, testimony, witness.

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3144 martus (mar'-toos);

of uncertain affinity; a witness (literally [judicially] or figuratively [genitive case]); by analogy, a "martyr":

KJV-- martyr, record, witness.