Cara Delevigne -- I Have a Pet Name For Sienna Miller

Cara Delevigne was coy about that pic that was snapped a few days back with her and Sienna Miller ... but our ears perked up when she told us about the nickname Cara gave her.

Cara was in a chatty mood as she left JFK ... but she tightened up when our photog asked what's up with her and Sienna Miller.

The 21-year old model then revealed the nickname ... but we couldn't figure out what it means, no matter how much we googled.

If you have an idea, sound off in the comments.