What is shroomin?

More than likely, it means "tripping" (being intoxicated) on magic mushrooms. Magic mushrooms, or "shrooms" for short are hallucinogenic mushrooms with effects similar to LSD. Supposedly the high feels more "natural" and less jarring than LSD.

There is a word of warning. Magic mushrooms are illegal in most places. Also, they look much like deadly mushrooms and grow in roughly the same places, so there have been many mushroom identification mistakes and deaths.