What does 'he's been living under her mattress' mean?

"He's been living under her mattress" is an idiom that can describe covert residency, extreme closeness, surveillance, or an unconventional arrangement, depending on context. In literal terms, it may refer to someone hiding on a mattress in a spare room, basement, or storage area within her home, often linked to situations involving stalking, abuse, financial hardship, or clandestine stays. Figuratively, the phrase can imply an intimate but concealed relationship, a person who is emotionally dependent, or someone enduring stressful conditions in a space they associate with safety. The following sections outline literal and metaphorical interpretations, reasons such behavior might occur, and how to assess and respond depending on the circumstances.