slight offense or sin
She regarded her husband's smoking as a mere peccadillo, but his drinking as a really dangerous bad habit.
peccare (to miss, mistake, make a mistake, transgress, offend, sin) (Latin)
"pecc" (actually, pecado) refers to "mistake" as in impeccable. pecking others etc. , So, peccadillo means small mistake. no one is PUKKA. we tend to do small and petty mistakes pecc (mistake) + dillo (dhila)means loose, so small mistake.