v.1. To bend at the waist suddenly, out of pain or intense laughter: We doubled up in laughter watching the old movies.
2. To cause someone to bend at the waist suddenly, as out of pain or intense laughter: That joke absolutely doubled me up!
3. To arrange something in pairs: We doubled up the children in the downstairs bedrooms.
4. To be arranged in pairs: The children doubled up, and each pair took a turn on the swings. If the crew doubles up, they can finish this job in half the time.
5. To do or use twice as much of something as usual: I didn't go to the gym yesterday, so I'm doubling up on my workout today. The clothes are filthy, so double up on detergent.
6. To share between two people something meant for one: There aren't enough beds for everyone, so they had to double up.
7. To perform two functions simultaneously: The new band has a lead singer who doubles up on keyboards. My key ring doubles up as a bottle opener.