\hss ¶Synopsis:
\hss
Produce a horizontal space that is infinitely shrinkable as well as
infinitely stretchable (this command is a TeX primitive). LaTeX
authors should reach first for the \makebox command to get the
effects of \hss (see \mbox & \makebox).
Here, the first line’s \hss makes the Z stick out to the right,
overwriting the Y. In the second line the Z sticks out to the left,
overwriting the X.
X\hbox to 0pt{Z\hss}Y
X\hbox to 0pt{\hss Z}Y
Without the \hss you get something like ‘Overfull \hbox
(6.11111pt too wide) detected at line 20’.