Have you seen your User Settings? There is a Show quips on XP loss/gain messages option that triggers such messages.

D'Oh! I was aware of the quips. (I didn't really know I could disable them, although it was easy to suspect that, but that doesn't matter because they do not annoy me.) Only, I didn't realize this was one.

On a largely OT, I presume, basis: anyone care to explain it? I just can't make sense of it, possibly due to English not being my mother tongue. An USA/UK biased cultural reference, maybe? (Or am I just that ignorant?)