IPA: /rɪˈzɜːrvɪst/
KK: /rɪˈzɜrvɪst/
A person who is part of a military reserve force and can be called to active duty when needed.
He is a reservist who trains regularly in case he is needed for service.
Reservist → It is formed from "reserve" (from Latin *reservare*, meaning to keep back or hold) and "-ist" (meaning a person who practices or is concerned with). A reservist is a person who is kept back for military service when needed.
Think of someone who is 'kept back' ('reserve') for military duty — that's what a reservist is.