Once, Tsar Alexei Mikhailovich ordered that a box be placed near his palace in Kolomenskoye for those who wished to write personally to the tsar.
The idea was to receive complaints bypassing various ranks of clerks.
Complaints poured in abundantly, so the box installed was large and deep, hence it was called long, as they said at the time.
But the petitions mainly came from ordinary people, the poor, and all sorts of wronged individuals.
And the complaints matched – some were beaten by boyars, some had their land taken away, others were robbed by officials.
For the tsar, these were trivial and boring, so he started to neglect these requests and forwarded them to various agencies, which also dragged their feet and sometimes took years to consider the petitions, and often lost them.
When the complainants realized that their writing was futile, they began to send abusive letters, which couldn't even be shown to the tsar.
So, the box was removed from sight, and the expression "to put in the long box" acquired its modern meaning.