The term "ate up" is typically used to describe people who have taken to many drugs. Marijuana makes you "ate up" over an extended use. The THC attatches to your fat cells mainly around your brain. Over time this blocks signals being sent from your brain making you "ate up." I would say Acid causes you to be "ate up" much quicker. This is based on personal experience and my drug class teachers...
Signs of being "ate up" are fairly simple.
1. You laugh at nothing... sober
2. You stuggle putting sentences together
3. Zone out of most conversations with a dumb look on your face
Man jake is so ate up from all that acid he cant even speak!