A.T. Robertson Commentary Hebrews 12:21

"and so fearful was the appearance, [that] Moses said, I exceedingly fear and quake:" — Hebrews 12:21 (ASV)

Fearful (φοβερον). As in 10:27,31, only in Heb. in N.T.

The appearance (το φανταζομενον). Present passive articular participle of φανταζω, old verb from φαινω, to make visible, here only in N.T. "The manifestation."

I exceedingly fear and quake (εκφοβος ειμ κα εντρομος). "I am terrified (εκφοβος, late compound like εκφοβεω, to frighten, Mr 9:6) and trembling" (εντρομος, late compound like εντρεμω, to tremble at, as in Ac 7:32; 16:29). Εκφοβος is quoted from De 9:19.