FROM STRONG’S GREEK DICTIONARY (1890)

Strong’s Greek Dictionary (1890) — public domain, printed whole.

seio σείω — si'-o — apparently a primary verb; to rock (vibrate, properly, sideways or to and fro), i.e. (generally) to agitate (in any direction; cause to tremble); figuratively, to throw into a tremor (of fear or concern):--move, quake, shake.