closely besetting: (of sin) surrounding and clinging to a person like a loose garment that wraps around a runner's legs and trips him
This word appears only once in the whole Bible, in Hebrews 12:1, and the writer of Hebrews seems to have coined it for that one sentence. The verse pictures the Christian life as a footrace and urges the runner to throw off "the sin that so easily besets us." Because it is a made-up compound, its exact force has to be reconstructed from its parts and from the race image around it.
The most widely held reading takes it actively: sin closes in on the runner and clings to him, like a loose robe wrapping around the legs and tripping him. The same verse tells the runner to strip off both this sin and every "weight" (ὄγκος); the sin is the more dangerous of the two, because it does not merely slow the runner but actively wraps him up, much as a person gets tangled in competing pursuits (ἐμπλέκω).
Underneath the picture of entangling, many scholars also hear a fainter note of "trouble" or "distress," since the word is built on a stem that elsewhere names a hard circumstance or calamity. On that reading the besetting sin is not only what trips the runner but what lands him in real danger. One early copy of Hebrews even has a slightly different word meaning "easily distracting," so that the sin pulls the runner's eyes away from the goal. These are not separate uses of the word, since it occurs only here; they are competing attempts to pin down a single, deliberately rich coinage.