Songfacts®:

• Perry told Rolling Stone magazine that this lament about a guy who's too sober and serious refers to an ex boyfriend. She explained: "Some people think it's a dis when I sing, 'You're not a man, you're a mannequin,' but it's not. Writing 'Mannequin' actually fixed some things in the relationship. It was my way of saying, 'I love you so much. Wake. The. F--k. Up. I want you to be in my life. Oh, and please stop wearing my jeans.'"
  
  A mannequin is a dummy used to dress up to display clothes, so in essence, Katy Perry is accusing this guy of being a dummy.
• "Mannequin" is part of Katy Perry's 2008 breakthrough album One Of The Boys, which has two vivacious hits - "I Kissed a Girl" and "Hot n Cold" - that established her as a pop star. What's really surprising is, seven years earlier she was a gospel singer adhering to her conservative Christian upbringing. As she got out into the world and met new people her worldview changed and she embraced secular music and culture. Perry grew to become one of the biggest stars of the 2010s with an impressive run of hits that includes "California Gurls," "Dark Horse" and "Roar." Even more impressive, she had a hand in writing all these songs.
• The term mannequin isn't used much outside the fashion industry so it doesn't show up in many songs, but a few months after Perry released "Mannequin," Britney Spears put out a song with the same title on her album Circus. It's like how we had two movies about an astroid apocalypse in 1998: Deep Impact and Armageddon.
  
  Worth noting, there was a popular movie Mannequin in 1987 starring Kim Cattrall of Sex And The City. The soundtrack includes the #1 hit by Starship, "Nothing's Gonna Stop Us Now."