adj. & n.
adj. 1 Math. (of a number) irrational. 2 Phonet. (of a sound) uttered with the breath and not the voice (e.g. f, k, p, s, t).
n. 1 Math. a surd number, esp. the root of an integer. 2 Phonet. a surd sound.
[ L surdus deaf, mute: sense 1 by mistransl. into L of Gk alogos irrational, speechless, through Arab. jadr asamm deaf root ]