To have sand (literally or metaphorically) stuck in your nether regions. Remember that time you were at the beach, and your bikini bottom slipped as you fell into the sand, and all those little pieces got stuck in your lady parts? Remember how miserable and bitchy you were? Remember that stupid cow from work, how she complained non-stop about stupid things, and got offended over every little comment? She had "sand in her taco." It was almost as if she had fallen into the sand and was miserable the rest of the day.
She is always so uptight; she has such a sandy taco