restraint; being absorbed in something
Rare literary noun. Can mean both external restraint and being so engrossed in something that one is metaphorically bound by it. The second gloss is an extended, less common usage.
There was no restraint on his actions.
I was so absorbed in reading that I lost track of time.
The kanji 牽 means 'pull' or 'tug', and 束 means 'bundle' or 'bind'. Together they suggest pulling together and binding, hence restraint or being tied up. The exact historical derivation is uncertain; the word is rare and likely a literary coinage.