To be shifty is to be sneaky and devious. Shifty behavior is untrustworthy.
Sometimes people shift around when they're nervous. That can help you remember the meaning of shifty, which describes someone who may not be literally shifting around, but they’re acting in a way that makes you think they're up to no good. Lying is shifty behavior. A fake ID is shifty. Asking a lot of questions about where money is kept is shifty too. Shifty actions and words are deceitful or suspicious. When someone acts shifty, others distrust them.