Eyes full of adultery
Here’s what it means
The person continually looked at others with adulterous desire and treated them as possible partners in sin.
Why the Bible says it this way
The writer deliberately uses very strong language to make an important point. The point is serious even though the wording is not meant literally.
A helpful detail: The eyes do not literally contain adultery. Peter describes a settled pattern of lust, not an accidental glance.