Definition from Wiktionary, the free dictionary
arrowslit (plural arrowslits)
- A slit, as in a castle wall, through which arrows can be fired.
- 1949, George Orwell, Nineteen Eighty-Four, Chapter 3, 
- When they met in the church tower the gaps in their fragmentary conversation were filled up. […] They sat talking for hours on the dusty, twig-littered floor, one or other of them getting up from time to time to cast a glance through the arrowslits and make sure that no one was coming.
- 1949, George Orwell, Nineteen Eighty-Four, Chapter 3,