I often play Nethack as a monk, so wisdom is a fairly important attribute. Occasionally I will see the message:
You feel foolish! You haven't been paying attention.
...which means my wisdom has decreased. The trouble is, I usually get these messages while I'm just walking around, so I haven't been able to match the wisdom decrease to a particular action or behaviour so far.
The wikis are a little vague on what, exactly, abuses wisdom. What could I be doing to trigger this?
The wiki has plenty of information on this. Wisdom is generally very hard to abuse and very easy to exercise, but the most likely candidate for you is probably:
Being satiated as a monk
The other criteria are so esoteric (like "Being hit by divine lightning when attacking an aligned priest") that it's doubtful that you'd have gotten this far without noticing yet.
As a general rule, try not to be satiated. It also abuses dexterity (regardless of your class), and if you find a corpse you want to eat, it might be a little more dangerous to attempt doing so (although you're a monk, so that's not as much of a problem). As a monk, it's generally a good idea to only eat when you're Hungry or worse.
If you want to recover your wisdom in a current game, you could:
apply a noncursed unicorn horn, which cures attribute decrease,
repeatedly Engrave "Elbereth" (with a capital E), which will exercise wisdom,
or just play as normal, since many common actions (reading scrolls, casting spells, sacrificing a corpse, etc.) also exercise wisdom.
