Much like the Freshman 15 when a student starts college he gains 15 pounds, the Covid 15 is when people start to gain weight due to inactivity during the Coronavirus pandemic.
When two individuals interact in sexual intercourse manner. Where one or both parties decided to cough on each other. Neither party needs to be covid postive, but it is recommended. As it heightens the risk. Thus involving both or multiple individuals in the Covid fuck.