Restrained
Part of speech: adjective
Definition: Held back, limited, kept in check or under control.
Part of speech: verb
Definition: To control or keep in check.
Example sentence: If the Italian is the most passionate lover in the world, it may be because he is the most restrained. Rigid convention denies him all contact with the lovelier girls, who never are free from chaperons.
Reticent
Part of speech: adjective
Definition: Keeping one's thoughts and opinions to oneself; reserved or restrained.