Letters that make hush-hush
s, h, u are letters that make 'hush-hush'
1 possible definitions for 'hush-hush'
Under this definition, hush-hush is an adjective and has the following properties:
clandestine, secret, cloak-and-dagger, hole-and-corner(a), hugger-mugger, surreptitious, undercover, underground have the same meaning as 'hush-hush'
underground, resistance are derivatives of 'hush-hush'.
covert is similar to hush-hush.