Dictionary entry

λάθρᾳ

G. Abbott-Smith's A Manual Greek Lexicon of the New Testament

λάθρᾳ (Att.; in Hom., -ρῃ, Rec. -ρα),

adv.,

[in LXX chiefly for ‎בַּסֵּתֶר;]

secretly: Mt 1:192:7, Mk 5:33 (WH, mg.), Jo 11:28, Ac 16:37.†