What is the safest, quickest way to gain weight?

Doctor can evaluate: If a person is truly underweight (based on one's height, family background, and body type), he should be evaluated to see if there is a medical cause for being underweight. If no medical cause is found, then he can improve his weight by eating more calories of nutritious foods, while also doing an appropriate (not an excessive) amount of exercise to build more muscle.

Get medically : cleared first. Eat healthy foods high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8 to 9 hrs/night. Hydrate well w water. Foods to help w weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola, trail mix, dried fruits, protein shakes & bars