Spike was born William, which in England is the same as the Irish Liam. The name means "protector".

William was known, unfavorably, as William the Bloody in his lifetime because of his bloody-awful poetry. He kept the name after being turned, by torturing his victims with railroad spikes.

Shortly after being turned William took the name Spike. The most common theory that has come up in canon as to why is that a party goer before William was turned said he'd rather have a railroad spike through his head than listen to William's poetry, one presumes William obliged after being turned. He has been referred to as William by Buffy, Drusilla, and Angel after taking the name Spike.