Being stunned means you have trouble walking in the direction that you intend - if you press a movement key you will always move, even if the direction attempted is blocked, say by a wall. There is a high chance that it will usually be in a random direction rather than the expected one. If your pet is next to you, you might attack it by accident; you can anger peaceful monsters this way.
Attempting to zap a directional wand while stunned will frequently cause the zap to be in a random direction. This applies to firing too.
Curing Stunning Edit
- Apply an uncursed or blessed unicorn horn
- Eating a lizard corpse will set stun counter to 2
- Getting blinded by an Archon while not wearing the Eyes of the Overworld will set stun counter to 1 to 3 
- Getting stunned by a monster casting stunning while having magic resistance or free action will set the counter to 1 
- Casting a forgotten spell will sometimes set the stun counter to 2, 4, or 6 for a 1st level spell 
Stunning has a less significant effect on monsters than on the player. It has the following effects:
- You (and only you) receive a +2 bonus to hit stunned monsters.
- Stunned monsters cannot block or evade your kicks.
- Stunned monsters are immune to your gaze attacks.
- Stunned monsters are less likely to jump out of the way of a falling portcullis or off a closing drawbridge.
- Stunned peaceful humanoids react differently when you #chat with them.
- Stunned spellcasting monsters are more likely to cast undirected spells, just as they would if they weren't adjacent to you. If they do this, they still get their normal turns, however.
Stunning does not cause a monster to move randomly or miss turns.