"Sub-bole" or delicious understatement!

Ah yes, we have all done it. "How are you?" someone asks, when we are steaming mad, and we say something like "oh, a little perturbed." Huh? Or, we have just seen a movie we loved, and respond to an inquiry about it with "not bad" or something equally milque-toasty.

Why? Well, let's see: sometimes we want to hide information from the asker of the question, or otherwise not let them know the extent of our enjoyment or displeasure. Sometimes we may be feeling the need for privacy. Sometimes we may deliberately want to mislead, for a variety of different reasons.

Any other ideas?