When you finish having sex with a girl and realise post-coitus the condom has been left inside her. In a haze of panic and confusion you use your index and middle finger to fish it out in a semi-sensual style. What you end up pulling out should look like a shrivelled old jellyfish, hence the term "Jellyfishing".
When Matt realised his vigorous love making and oversized condom had resulted in the rubber slipping off, he kissed Lisa while jellyfishing.
He eventually came up with the goods and then prayed she was not pregnant for the next few months.