Holds a few meanings. And was established at Western Illinois University c. 2008

A situation, comment or story that makes you laugh as if you were high.

Something that happened while you were high that was so funny that while reminiscing about it sober, makes you laugh as if you were high.

When in public conversing with some one you know is down, in front of others who are not down, it is a code to let them know that the particular funny instance happened while being high.

HIGH-lar-i-us (adj): A particularly clever yet unpretentious joke or situation which does not readily present itself as humorous absent higher intellectual processing. The rare occurrence where a British-style pun is actually quite funny. This definition, however, explicitly excludes esoteric, disciplinary humor such as that shared amongst PhD candidates, law-students and anyone remotely associated with the New Yorker.

adverb. highlariously
adj.1) a high level of lariousism
2) something that is hilarious whilst high in an altered state of consciousness

When something that usually wouldn't be funny is funny, but only because your high.

Something that is only funny when you are stoned. Can also be substituted with highsterical. The more stoned you are the more highlarious things seem to be.

Something that it extremely when/because you are high.

Something that is only funny when you are high.