Latin for "completely untrustworthy" (usually said by the new guy in the office who wants to impress management). Similar to "on the fly" but implies a level of intelligence is required to do it and to understand its output.
"I've just done some ad hoc analysis of your numbers and they don't seem to add up". "I've made some ad hoc changes to the website - everything's fine now". "Can you produce an ad hoc report to shut Julian up at the next ops meeting?