纠结 literally means 'to twist together' (like two ropes), but its far more common modern use describes a mental state: feeling conflicted, torn between choices, or obsessively hung up on a problem. You can say 纠结于某事 (to be tangled up by something) or use it as a stative verb in 心里很纠结 (one's heart is tied in knots). The archaic sense 'to band together (with illicit intent)' is rare in contemporary Chinese.