What

An irrefutable claim that one's ex is currently engaging in monogamous bliss with someone else, often shouted with the intensity of a porn star at a strip club. Conveys both jealousy and disbelief at the audacity of their ex's 'new' relationship.

Examples:

• " When I saw Sarah happily posting a sunset with her new boyfriend, I blasted 'Her's is taken!' across the college quad "
• " The guy in the office yelled 'HER'S IS TAKEN!' after overhearing his ex's partner whisper sweet nothings into their ear "
• " I screamed 'HER'S IS TAKEN!' while watering the plants, causing everyone to think I'd finally found my true love "