self-indulgent
擅 centers on acting solely according to one's own desires without restraint: self-indulgence.
擅 combines 扌 (hand) with 亶, which likely contributes the sound and may relate to acting freely or monopolizing. The exact historical development is uncertain.
A hand (扌) beside 亶: imagine a hand reaching out to take whatever it wants, indulging every whim without restraint.
For セン, picture a self-indulgent person spending a thousand (千, せん) yen on a whim: 千 -> セン, and the hand grabs the money.
field in which one acts unchallenged; unrivaled sphere of activity (unrivalled); one's monopoly