By the year 1998 I was using the term "shard" in slides for MySQL, and I believe mSQL as well. The term might be found in Slashdot's source code. I am pretty sure I gave a talk on sharding at the open database summit in 2001.
I had previously used the term shard for an app I wrote and distributed for VMS around '92, but that app only ever had a handful of users. It was available via FTP.
FWIW I took the term from the movie "The Dark Crystal".