IPA: /ˈmɪs.ɪ.lɪr/
KK: /ˈmɪsɪlɪr/
A person who is involved in the operation, maintenance, or management of missiles.
The missileer was responsible for ensuring the missile systems were ready for deployment.
Missileer is derived from "missile" (from Latin "missilis", meaning capable of being thrown) and the suffix "-eer" (meaning a person who). The term refers to a person who operates or is involved with missiles.
Think of someone who is involved with something that is 'thrown' ('missilis') — that's why a missileer is a person who operates missiles.