Jack Frost
Jack Frost has visited means every thing is freezed due to extremely cold winter season
Jekyll And Hyde
Used for people who have both sides of character – A pleasant and a very unpleasant side.
Judge, Jury And Executioner
When someone has the overall authority to take decision and can do whatever he/she wishes we use this idiom.
Juggle Frogs
When some one tries to do something which is extremely difficult we say he is juggling frogs.