What is "placket hole"

Placket \Plack"et\, n. [F. plaquer to lay or clap on. See Placard.]

1. A petticoat, esp. an under petticoat; hence, a cant term for a woman. [Obs.]
   --Beau. & Fl.

2. The opening or slit left in a petticoat or skirt for convenience in putting it on; -- called also placket hole.

3. A woman's pocket.