"Coltre or Coltri" is a word nowadays little used, which in Italian means "the blankets".
This is an easy-to-make disguise, maybe not totally practical for going around but, trust me, February is usually a pretty cold month in Venice, a blanket may save the day sometimes.
And here in the image we have a clear explanation of the disguise:
“Donne ne la Coperta io stò celato
Perche del fornicar son il peccato”
“Oh women, I am hidden in the blanket
Because I am the sin of fornication”
I would say this is just a funny - and quite obnoxious - way of teasing women. telling them the “how good I am in sex” old story.