A dirty bulk is a muscle-building phase where you eat far more than you burn without much of a plan, often on the principle of "just eat more". Its counterpart is the lean bulk, with a small, planned surplus.
The logic sounds reasonable: more energy, more growth. But muscle grows slowly, and your body does not build it any faster just because extra energy is available. Whatever goes beyond what muscle growth needs mostly ends up in fat tissue.
What a study shows
Garthe et al. 2013 compared elite athletes on the same strength program for 8 to 12 weeks. One group ate about 3,585 kcal a day on average, the other about 2,964 kcal. The higher-calorie group gained clearly more weight, 3.9 versus 1.5 percent of body weight. But their fat mass rose by 15 percent instead of 3 percent, and there was no difference in fat-free mass. The extra 600 or so kcal a day mainly bought fat.
You pay for it in the cut
Every kilogram of fat from a bulk has to come off again later. Iraki et al. 2019 point out that too much fat gained in the off-season makes the following diet longer or harder. On top of that, an analysis by Forbes 2000 suggests that the more body fat you already carry, the smaller the share of a surplus that becomes fat-free mass.
So if you want to gain fast, check your rate first. If your trend weight climbs clearly faster than about 0.5 percent of body weight per week for several weeks, you are closer to a dirty bulk than a lean one.