Dictionary entry

μάτην

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

μάτην

(prop. acc. of μάτη, a fault, folly),

adv.,

[in LXX for הֶבֶל, שָׁוְא, etc.;]

in vain, to no purpose: Mt 15:9, Mk 7:7^(LXX).†