See the verb espier. Compare also Italian spia, Spanish espía.
espie m (oblique plural espies, nominative singular espies, nominative plural espie)
circa 1170, Wace, Le Roman de Rou:
- Dez ke Richart le sout, un espie enveia
Saveir u Thiebaut ert, e combien gent il a.
- As soon as Richard knew about it, he sent a spy
to know where Thibalt was, and how many people he had with him.