Jay is one of those words that gets used for slang constantly in all sorts of contexts.

1. These days, jay is most often used as slang term for marijuana, or a cigarette made from marijuana. Jay Purple is slang for ketamine.

2. Jay can also mean 'to steal'.

3. Or a person who to talks or gossips excessively; this may come from the bird jay (see Webster), which can be rather loud and long winded.

4. Jay is also an older slang term (I can't find when it came into popular use, but it was still used in the 1920s) for a chump, fool, or sucker.