Kålmask is ’caterpillar’or ’larva’ of kålfjäril, Pieris brassicae, ‘large white’ or ‘cabbage butterfly’. (Kål is cabbage, a loan from Old English cawel (English cole, Scottish kale) from Latin caulis with the same meaning.)
Mask is ‘worm’ but larvae are also often called mask, though it is not scientifically correct. It is a common Germanic word, the English version is maggot. There is a homonym; same as English mask, from French masque from Medieval Latin masca, ‘mask’,‘ghost’ ‘nightmare’.
Latin larva also means ‘ghost’, ‘evils spirit’ and ‘ mask’. The Swedish loanword larv (via German) first meant ‘disguise’ or ‘hideous mask’; Now it has two meanings ‘caterpillar’ and ‘nonsense’, ‘silly behaviour’. When Linnaeus needed a scientific Latin word for an infant insect he took larva. Maybe he made a pun!
Caterpillar comes from Old French chatepelose, from Medieval Latin catta ‘female cat’ and pilosus ‘hairy’.
The wether is still mild; today I found a couple of larvae on the Swedes I should have lifted long ago. (Yes, Swede or Swedish turnip is kålrot). Then I saw one larva and a pupa on the underside of the eaves of an outhouse. Probably they are going to spend the winter there.