Expression often used when a person doesn't immediately react emotionally to news or events. It took a moment for the gravity of her achievement to sink in.
Pro Tip 2/3
Informal Usage
Common in casual conversation, less appropriate in formal writing. Give them time; the news will sink in eventually.
Pro Tip 3/3
Metaphorical Sense
Used figuratively, not describing physical movement but mental processing. Her words began to sink in, and I started to comprehend the lesson.