Ian Anderson didn't pick the name
Jethro Tull, his
agent did, and it was for
no other reason than that they needed a name for that
night in order to get re-booked at that club. Their agent had studied
history and Jethro Tull was the name of the inventor of the
seed drill as others have mentioned above.
That same
night the club owner offered them a regular spot at the
club and by that time it was too late to change their name.