Sadly, in posse doesn't involve being a member of a Wild West gang:
the Ned Christie posse
which is intent on catching a villain in a black hat* (which must be quite good fun, and doesn't even require very much detective work unless the said villain is smart enough to, well, take his hat off - which, as far as I have observed, very few of them do. I suppose it's analogous to some of our local English villains who have HATE and KILL tattooed on their knuckles.).
No, in posse means possible or potential. In Latin.
And why anyone would choose to use it I have no idea at all.
Sunday Rest: in posse. This is Latin for in possibility. Lawyers use the phrase from time to time, and the example usually given is of a foetus being a child in posse, that is a potential child, rather than in actual being (in esse).
But then some lawyers are dreadful show-offs, aren't they.
*Not that Ned Christie wore a black hat: though some of the members of the posse do. But then they were the baddies, weren't they?