jell out · verb1
Uncertain. Plausibly a eggcorn from, or a humorously intentional alteration of, chill out. Alternatively, perhaps a figurative extension from the gelatin-related sense, as gelatins are allowed to rest while they set up, and their setting up involves stasis and calm.
jell out · verb2
Phrasal verb with particle, denotatively equaling the nonphrasal counterpart (in at least one sense or subsense thereof), which is a phenomenon with many instances (for example, dry out and dry, shrivel up and shrivel, sit down and sit).
Etymology from Wiktionary, CC BY-SA 4.0.