At training this year at TITP I heard the term pettiskirt. I assumed it was supposed to be the same as a petticoat. I checked with Elizabeth Stewart Clark and it's not a term she's run across in period sources. More common terms for a petticoat, according to Liz, would be underskirts, or even just skirts (The Dressmaker's Guide, 153). Perhaps "pettiskirt" is just a combination of "petticoat" and "underskirt"?
For additional concise details on petticoats, see Liz's FREE Basic Petticoat Pattern as well as my Petticoat post.