Lexical Definition

‎στυγνάζω‎ stygnázō (stoog-nad'-zo) be gloomy ^G:V
from the same as ‎στυγνητός‎

lower, be sad.

• to render gloomy, i.e. (by implication) glower (be overcast with clouds, or sombreness of speech)

Abbott-Smith

† στυγνάζω (< στυγνός, sombre, gloomy, sullen, Is 5:17, Wi 17:5, Da LXX 2:12*), [in LXX: Ez 27:35 28:19 32:10 (שָׁמֵם)* ;]
to have a sombre, gloomy appearance: of the human countenance (RV, his countenance fell), Mk 10:22; of the sky (so στυγνότης, Polyb., iv, 21, 1), Mt 16:3 (|[WH]|, R, mg., om.).†

Abbott-Smith References

Liddell-Scott-Jones

στυγνάζω-άζω,

(στυγνάζωός) to have a gloomy, lowering look, ἐπὶ τῷ λόγῳ [NT+7th c.AD+]; of threatening weather, [NT]

_{LSJ Scripture Refs: Mark.10.22 ☩ Matt.16.3}