This inspired this question; I should verify whether the prefix really means something.
*defallere
from Latin de- "away" + fallere "to deceive, to cheat; to put wrong, to lead astray, cause to be mistaken; to escape notice of, be concealed from" (see fail (v.))
The purpose of de- here is unclear to me, because the semantic notion of "away" does not appear in the verb *defallere and because Latin already had fallere. So why might this prefix have been added to this word prior to entering Old French?