Brace:

When you enter the 'brace' command to brace your character, your character
will no longer move.  It can still attack adjoining spaces.  Bracing can
be useful to hold a location.

When you are braced, you lose your dex bonus and incur a 2 point
ac penalty beyond that (if you have a negative dex bonus, you may in
fact come out ahead.  You also only get 20% of the normal experience
for killing creatures, and incure a 4 point wc (to hit) penalty.