I. ˈən-dər-ˌstə-dē, ˌən-dər-ˈ
Date: 1874
intransitive verb
: to study another actor's part in order to substitute in an emergency
transitive verb
: to prepare (as a part) as understudy ; also : to prepare as understudy to (as an actor)
II. ˈən-dər-ˌstə-dē noun
Date: 1882
: one who is prepared to act another's part or take over another's duties