means Feeling resentful or anxious that something you value—a partner, a position, a possession—might be taken or shared with someone else.

from From the Greek 'zēlos,' meaning zeal or ardent rivalry, which traveled through Latin 'zelosus' into Old French 'jalous' before reaching English. Tellingly, 'jealous' and 'zealous' are the same word wearing different clothes—both descend from that root of burning eagerness. The original sense was fierce devotion; only later did that heat curdle into the suspicion we mean today.