1.  Cricket. An extra fielder sometimes placed to back up the two slips (short-slip and long-slip). b. His position in the field.

1

1891.  Daily News, 23 June, 3/7. He was let off … by Wilson at cover-slip.

2

  2.  A slip of thin glass used for covering a microscopical preparation; = cover-glass (COVER sb.1 8).

3