The file system specified after the first colon probably contains many small files, exceeding the per-file system limit for inodes (file information nodes).
Make several backup copies of the file system on different tapes (for safety).
Change the machine to single-user mode.
Use the newfs(1M) command with the -i option to increase inode density for this file system. The following is an example:
# newfs -i 1024 /dev/rdsk/partition
Restore the file system from a backup tape.
Increasing the inode density slightly reduces the total file system capacity.