Strong’s Greek
G3154 — mataioo
The entry below is Strong’s own definition (1890, public domain), and every verse where the word occurs.
ματαιόομαι
mataioo — pronounced mat-ah-yo'-o
from 3152; to render (passively, become) foolish, i.e. (morally) wicked or (specially), idolatrous:--become vain. see GREEK for 3152
1 occurrence in the KJV.
Romans 1:21
Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.