1
general::
Defer indefinitely or for a very long time, as in We've put the plans for a new library in mothballs. This expression alludes to storing woolen clothing or other items with marble-size balls of naphthalene or camphor to prevent them from being damaged by moths. [1940s]
American Heritage Idioms
2
general::
Phrase(s): put something in mothballs
1. Lit. to put something into storage in mothballs. • He put his winter coat in storage with mothballs each fall and had to air it out for a week each spring. 2. Fig. to put something into storage or reserve. (Often said of warships.) • The navy put the old cruiser in mothballs and no one ever expected to see it again. • Let’s just put this small bicycle in mothballs until we hear of a child who can use it.
McGrawhill's American Idioms And Phrasal Verbs