Round turn

From WikiPOBia

Specifically, a round turn is the passing of a line through 360 degrees around an object. The friction caused by the turn when such a line comes under tension is dramatic. Thus, in tending a line apt to come under heavy strain, a sailor will often "catch a turn" around a belaying pin, cleat, or bit to aid him in maintaining control.

A common example is, "a round turn and two half-hitches", a knot said to be "Good enough for the King's yacht"

Characters throughout the canon frequently speak of being, "brought up with a round turn". Maturin often mistakes the phrase. At one point, he tells Major Beck that he "brought him up with a round stern". At another he assures Aubrey that, "if the Admiral proves inquisitive, I may toss him off with a round turn." The nautical error, leaving aside the question of pun, is that a round turn helps to make something fast, not to let it loose.