Snug as a bug in a rug

This is a metaphor for being wrapped up, warm and cosy, rather like an insect feels when it finds a warm, cosy home in a small carpet. It is first cited from 1769 as a quotation from a play by David Garrick, the celebrated British actor, who wrote a play in honour of Shakespeare in which the expression was used. One assumes the expression was already in vernacular use before this.