Hi, we use this slang expression in Australia. It means that you are extremely tired/not functioning. It can also mean the person is dead. Example: I was working so hard today that I'm 'cactus' and I don't feel like going out anywhere tonight. eg the dishwasher was cactus so we had to wash all the dishes in the sink. When I went to wake him, he was cactus, so I called the ambos [ ambulance]

"prickly", "thorny", "stinging", extremely difficult, can be dangerous to touch, hold, or handle. If he is a cactus in the morning, maybe it's his beard, or maybe it's his disposition. Emotions can be touchy, sensitive, and the expression "He's a Cactus in the morning" does not have a positive connotation. I have never heard this expression used in conversations in California except to refer to needing a shave. He has a "five o'clock shadow". He needs to shave. Good Q!

Yes, we say "dead meat" as well as "cactus" in Australia. Seems this person will be in big trouble when you catch up with him! I can't improve on Jura's examples, they're great. :) And "cactus" can apply to anything that doesn't function, not only humans.