A wet burrito is a sexual act in which a man liberally applies hot sauce to his erect penis before inserting it into his partner's anus. The act results in a mixture of bodily fluids - fecal matter, blood, semen, sweat, possibly urine - and hot sauce, hence the "wet burrito" name. This is usually a surprise sex act, as most partners would prefer to keep hot sauce as far away from their genitals as possible.
You should've seen her face after that wet burrito I gave her last night. I have a feeling I'm not getting laid again for at least a week.