1) A white mark down the face of a horse.
1531 a blake stag stonyd with a white snype of the snote and a sterne in the forehead, Kirby Underdale
1588 one blacke meare with a white snippe in the snowte, Monk Fryston
1614 a bay mare with a starre and a snippe, Coxwold
1631 sould one blacke maire with a little white snip ... unto Wm Jennings, Adwalton. In 1626, Richard Brigham of South Cave bequeathed one blacke maire named Snippy to his son.
2) A narrow access which allowed people to slip from one part of the house to another.
The meaning is explicit in the only example noted: 1738 out of that snip or passage between Lord Wentworth’s room and the room above the parlour.