From a (as a negative particle) and a presumed derivative of kerannumi; unmixed, i.e. (figuratively) innocent -- harmless, simple.
see GREEK a
see GREEK kerannumi
a. unmixed, pure, as wine, metals,
b. of the mind, without admixture of evil, free from guile, innocent, simple: Matthew 10:16; Romans 16:19; Philippians 2:15; (and frequent in secular writings). (Cf. Ellicott on Philippians, the passage cited; Trench, § lvi.; Tittmann 1:27f.)