Show (one's) horns phrase
The devil is typically depicted with horns.
To display one's wicked, bad, or malevolent character.
I have never thought that a nice guy like John could be so wicked until he showed his horns by beating his girlfriend brutally.
I asked my girlfriend wether she still loved me if I had showed my horns.
After showing his horns, he was ostracized by his colleagues.
The verb "show" must be conjugated according to its tense.
The origin of this phrase is not clear.
White lie
Ken: Yesterday I lost the watch that my girlfriend presented to me. Should I tell her this thing?
Lily: Certainly Not! Your girlfriend will be angry with you. Children and fools tell the truth.