An interesting history for the word martyr:
From: English Words from Latin and Greek elements.
Ancient Greek legal procedure is reflected by the word martyr, which in classical times meant “witness” and was a perfectly ordinary word for a person who testified at a trial. The early Christians, however, used it in reference to those who bore “witness” for Christ and, because so many of them died for their faith, the word has come to mean “one who suffers in behalf of a cause”
Etymology: < post-classical Latin martyr witness, martyr (2nd–3rd cent.; used of pagan philosophers by St Jerome) < Hellenistic Greek μάρτυρ, variant of ancient Greek μαρτυρ-, μάρτυς witness (used in the New Testament of witnesses for the faith who suffered martyrdom, as St Stephen at Acts 22:20), perhaps a derivative of the Indo-European base of Sanskrit smṛi- to bear in mind, remember; but some scholars consider it a word of non-Indo-European origin.
The Greek word appears to have been adopted in Gothic, where a damaged reading is usually reconstructed as martwr . The post-classical Latin word passed widely into Romance languages (Anglo-Norman martir , Old French martir (c1050), Old French, Middle French, French martyr (13th cent.; also †martre ), Old Occitan martyr , martir (13th cent.; Occitan martyr ), Catalan màrtir (c1300), Portuguese mártir (1107), Spanish mártir (1207), Italian martire (a1294)), and Celtic languages (Early Irish martir (Irish martír , also martíreach ), Welsh merthyr , Middle Breton, Breton merzer ). Most Germanic languages show loans from the post-classical Latin word (Old Frisian martir , Old Saxon martir (Middle Low German martir , martire ), Old High German martyr , Swedish martyr , Danish martyr ), but others have as well as or instead of this a derivative (see -er suffix1) of the word meaning ‘martyrdom’ < post-classical Latin martyrium (see martyre n. and Germanic loanwords listed s.v.), hence Old High German martirere , marterere (Middle High German marterære , merterære , martelære , mertære , German märtyrer ), Middle Low German marteler , martelēr , martelēre , merteler , Middle Dutch martelāre , maertelāre , martelēre (Dutch martelaar ). Old Icelandic has instead the native formation píslarváttr , lit. ‘torture-witness’.
With sense 4 compare French martyr (1690 in this sense). (In quot. 1568 at sense 4a at this sense the word may show instead an otherwise unattested intransitive verb; compare Middle French martirer to be anguished.)