LOL desperation that is its origin not its meaning today, you said we use martyr from Christians that is incorrect I said the closest interpretation would be the English word martyr but that is not its literal definition.
What an earth, etymology is not desperation!
The etymology of the word "shahid" suggests that it primarily means "witness" and the same word began to be used by Muslims as anhonorific title for Muslim martyrs who died fighting in the way of Islam. Subsequently, and over a period of time, the same word was adopted by non-Muslims in the Middle East and South Asia such as Arab Christians and South Asian Hindus and Sikhs to denote their martyrs.
Interestingly, the English word "martyr" originates from the Greek word "martys", which also means "witness" in the Greek language. Therefore, in both the Arabic and Greek languages, the origin for the word "martyr" is "witness."