(1) Muharram is the first month of the Islamic Calendar. This month is considered a month of mourning by the Shia (and many Sunni) Muslims due to the murder of the Grandson of Prophet Muhammed (SAW), Imam Hussain (AS).
(2) Muharram can mean ‘made forbidden’.
Muharram is so-called because it was the main month designated by the pre-Islam Arabs as a month where war was forbidden. They didn’t usually follow their own rules and they even made an agreement where they would observe the law on year and the next year they could fight. They also broke this agreement too.