Maybe it's already obvious but hack is not a Cajun word. It's an English word. Or at least in the southern United States. Jimmie Rodgers used the word in Frankie and Johnny. It just sounds like a wagon.

Bring out your rubber-tired buggy
Bring out your rubber-tired hack
I'm taking my man to the graveyard
And I ain't gonna bring him back
'Cause I shot my man
He was doing me wrong

Yeah! That's it! I knew I heard that word before somewhere.
I know it from a Charlie Poole song. (Frankie & Johnnie a.k.a. Leaving Home)

Send for your rubber tired hearses
Send for your rubber tired hacks
Carry my Johnnie to the graveyard
I shot him in the back
...