In fact, hocus pocus does not appear to derive either from hoc est enim corpus meum1 in the mass, nor from hic opus, hoc labor est2, which is in fact from book 6 of Vergil's Aeneid, and describes the escape from hell.

It's from hax pax deus adimax3, a sixteenth-century quack-doctors' chant used to deceive the illiterate populace. The etymology site is as unenthusiastic about 'hoc est corpus' as my other sources, but offers 'hocus pocus, tontus talontus, vade celeriter jubeo'4 as a seventeenth-century juggler's patter. The modern word hoax may derive from 'hocus pocus', as may the nineteenth-century expression 'holus bolus', meaning 'all at once'.

Sources: Phillip Howard, Words Fail Me, chapter 14: 'Lemmings'. Also found in The Concise Oxford Dictionary of English Etymology.

1: 'For this is my body'
2: 'Here is the task, and this is the labour'
3: Gibberish: 'hax' is meaningless; 'pax' means 'peace'; 'deus' means 'god'; and 'adimax' looks like part of the verb adimere meaning to throw away, but isn't.
4: More gibberish. 'Vade celeriter jubeo' means roughly 'I order you to get out of the way, fast', but none of the other words are Latin at all.