Timid, timorous both mean so fearful and apprehensive as to hesitate or hold back.
Timid stresses lack of courage and daring and usually implies extreme cautiousness and fearfulness of change or of venture into the unknown or uncertain.
Timorous stresses domination by fears and apprehensions; it implies a temporary or habitual frame of mind which causes one to shrink from an action or activity which requires independence, decision, or self-assertiveness and suggests terror rather than extreme caution.