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.