Definition of hardens:
(v) :
(transitive, ergative) To make something hard or harder.
(v) :
(intransitive) To become hard.
(v) :
(transitive, figurative) To strengthen.
(v) :
(ambitransitive) To become or make (a person or thing) resistant or less sensitive.
(v) :
(transitive, computing) To modify (a website or other system) to make it resistant to malicious attacks.
(v) :
(intransitive, informal) To get an erection.