xyster (n.)

1590s, "compact, trim" (of a ship), especially "protected from the weather," perhaps from a Scandinavian source such as Old Norse snoggr "short-haired," Old Swedish snygg, Old Danish snøg "neat, tidy." This is perhaps ultimately from PIE *kes- (1) "to scratch" (see xyster).

The sense of "in a state of ease or comfort" is recorded by 1620s. The meaning "fit closely" is by 1838. Expression snug as a bug in a rug is attested by 1769; earlier was snug as a bee in a box (1706).