From a (as a negative particle) and sphallo (to "fail"); secure (literally or figuratively) -- certain(-ty), safe, sure.
see GREEK a
a. firm (that can be relied on, confided in): ἄγκυρα, Hebrews 6:19 (where I, and Tr have received as the form of accusative singular ἀσφαλην (Tdf.7ἀσφαλην; cf. Tdf. at the passage; Delitzsch, commentary at the passage) see ἄρσην). Tropically, certain, true: Acts 25:26; τό ἀσφαλές, Acts 21:34; Acts 22:30.
b. suited to confirm: τίνι, Philippians 3:1 (so Josephus, Antiquities 3, 2, 1).