I saw this question asked on the maemo forums while looking into pentesting apps for the Nokia n900.
I thought this answer by a user called brendan was brilliant:
“pentesting is a term for the QA team under the employ of companies like Bic and PaperMate, that scribble with the pens coming off the production line, to ensure that each one works before it is packaged and shipped to retail stores.
a mundane and monotonous job, but someone has to do it.”
Now in the future I know what to answer when someone asks me!
If anyone does want a real definition please see http://en.wikipedia.org/wiki/Penetration_test.