If the top word on the stack is non-zero (i.e. "true") then duplicate it. More massively-useful than it would appear, since it allows stuff like:
: countdown ( n -- )
begin ?dup while . 1- repeat cr ;
to print the
numbers from n down; here if we'd used
DUP instead of
?DUP, we'd have to
DROP the extra
0 value generated on a loop exit somewhere after the
REPEAT.