Used other than figuratively or idiomatically: see hard, nut, crack.
A problem that is challenging to solve.
A situation, person, group, etc. which is difficult to overcome or deal with.
A place, opportunity, etc. to which it is difficult to gain entry.
An amount that is difficult to finance.
Something secret or unexplainable; an unknown.
Someone or something with an obscure or puzzling nature.
A religious truth not understandable by the application of human reason alone (without divine aid).
A particular event or series of events in the life of Christ.
A sacrament.
A secret religious celebration, admission to which was usually through initiation.
A craft, art or trade; specifically a guild of craftsmen.