What does "habeas corpus" mean?

It means 'you have the body'. A writ of habeas corpus is a judges order to a prison official ordering that an inmate be brought to court so it can be determined if he or she is being legally held.

It actually means 'you shall have ( i.e may take) the body'. To obtain the order an application was made on behalf of the prisoner. The successful applicant got the Court's order saying, in law Latin ,just that, which he then showed to the jailer who then would release the prisoner to him. This is essentially what happens now . The writ is still called by the original Latin words of the order sought.

Which reminds me of the phrase, "corpus delicti," which many people take to mean the body in a murder case. Instead, it is the case itself - the evidence. Now, don't say I never told you anything worthwhile. Catty

Yes, Cattywampus, 'corpus delicti' has been misunderstood; most notably by John Haigh a serial killer in Britain in the late 1940s. He had read about the 'corpus delicti'. He procured enormous amounts of industrial acid and a vat, convinced that if there was no body (corpse?) found no murder charge could be proved. Then he set about killing victims for profit, forging powers of attorney, claiming insurances, stealing their effects etc, (he even kept the pet dog of one!), and dissolved the corpses, later pouring the sludge on to waste land.. The police only found odd bits of a foot which fitted a victim's shoe, though some personal items and some dentures (!)proved reasonably identifiable. Too late was he enlightened, to his shock; so then he claimed insanity. The jury found that he knew what he was doing all right, and why ( no surprises there) and he was hanged. If only he'd learned Latin properly his victims might have lived ( and so, therefore, might he!)