Something/someone consistently on one's mind.
I have the biggest crush on them, they lives in my brain rent free.
In recent years, the phrase living "rent-free" in one's head has gained popularity, appearing frequently across various social media platforms. Its origins can be traced back to the literal concept of living somewhere without paying rent. Although this arrangement benefits the tenant, it does not benefit the landlord. People often use this phrase on social media to describe someone or something that persistently occupies their thoughts without offering any tangible benefits. For example, a person might comment on how they keep replaying an embarrassing situation, saying it lives in their head "rent-free." This modern usage captures the idea of intrusive thoughts that linger without adding any value to the person experiencing them.