What does appall mean?
The definition of appall
Ap*pall”, v. t.
[imp. & p. p. Appalled; p. pr. & vb. n. Appalling.]
Etym: [OF. appalir to grow pale, make pale; a (L. ad) + pâlir to grow pale, to make pale, pâle pale. See Pale, a., and cf. Pall.]

1 To make pale; to blanch. [Obs.] The answer that ye made to me, my dear, . . . Hath so appalled my countenance. Wyatt.
2 To weaken; to enfeeble; to reduce; as, an old appalled wight. [Obs.] Chaucer. Whine, of its own nature, will not congeal and freeze, only it will lose the strength, and become appalled in extremity of cold. Holland.
3 To depress or discourage with fear; to impress with fear in such a manner that the mind shrinks, or loses its firmness; to overcome with sudden terror or horror; to dismay; as, the sight appalled the stoutest heart. The house of peers was somewhat appalled at this alarum. Clarendon.

Synonyms
— To dismay; terrify; daunt; frighten; affright; scare; depress. See Dismay.

Ap*pall”, v. i.
1 To grow faint; to become weak; to become dismayed or discouraged. [Obs.] Gower.
2 To lose flavor or become stale. [Obs.]
Ap*pall”, n.
Terror; dismay. [Poet.] Cowper.