A metaphorical and rather elusive phrase which is used to describe having obsessive thoughts over another person or group (usually followed by 'in my head').
The elusive part, imo, comes from the
phrase's seeming
implication that someone wants to live inside your head, as in they wanted you to think about them, when it's often the opposite case (i.e. you obsess about them and they don't know/it wasn't their plan).
So when I say "Jake is living rent-free in
Sally's mind", it doesn't imply that Jake has successfully
enamoured Sally with his charm. What it really means is that Jake is unaware that he is the object of obsession in Sally's mind. But since Jake is unaware, his pov becomes
nullified and therefore
defaults to Sally's pov ("Sally is obsessed with Jake"). This phrase 𝘤𝘢𝘯 be used to imply that Jake had successfully infatuated Sally, however most people tend to use it for its inverse meaning. It can also be used for someone beyond just your crush, such as someone who hates you.
Just shining some light on the hidden logic behind this phrase, which some people might find confusing (e.g. me).
Example:
𝗖𝗼𝗺𝗺𝗼𝗻 𝗘𝘅𝗮𝗺𝗽𝗹𝗲:
Maddy: Who are you staring at?
Ophelia: No one...
Maddy:
Damien's living rent free in your head, I can tell.
𝗢𝘁𝗵𝗲𝗿 𝗘𝘅𝗮𝗺𝗽𝗹𝗲:
Rust Teammate 1: Hey I was away for a few days,
fill me in.
Rust Teammate 2: We just pulled off a massive raid on one of the most heavily fortified bases, and we destroyed it to smithereens!
Rust Teammate 1: Damn! nice!! I guess we must be living rent-free in those team's heads now, hahaha.