You're thinking of $_ and !$, and I think they are equivalent - $_ is explicitly the last argument of the previous command, whereas !$ is a history expansion command. ! starts a new history expansion command, and $ is a word designator for the last word of the selected history entry. To quote man bash, " If a word designator is supplied without an event specification, the previous command is used as the event."
I would not that !$ is a bash-ism, whereas $_ works at least on zsh and ksh as well as bash.
I would not that !$ is a bash-ism, whereas $_ works at least on zsh and ksh as well as bash.