If you are having a bad day, say you are depressed or coming down with a cold, you could say, "I just don't feel myself today" and that would be just fine.
As for the "basser" as ZekeComa mentioned, there is the term busboy or maybe busser. A busboy is the next lowest job on the totem pole of jobs in the restaurant to a dishwasher. They clear and set the tables, maybe get water and bread or a clean utensil of the customer dropped theirs. They work for a portion of the waiter's tips.
In some restaurants, there is also a Food Runner. Their job is to ONLY take food from the kitchen to the table. They too work on a portion of tips from the waiter.