The build up of dead skin cells mixed with congealed underarm deoderant, particularly aluminium based preperations.
This mixture then permanently stains the arm pit area of business shirts leaving a yellowish stain on white business shirts. Ironing the shirt sets this mixture permanently.
Furthermore, this 'pit rott' contains an accumulation of bodily secretions giving a pungent aroma of concentrated stale urine, specially when the garment is being ironed or worn on a warm day.

When you eat dirt for the first time and leave your family to build a shack in the woods