to delay, to loiter, to hinder
躭 centers on lingering or holding back: delaying action, loitering without purpose, or hindering progress.
躭 combines 身 (body) with 冘, which likely contributes the sound and may relate to sinking or lingering. The exact historical development is uncertain.
A body (身) that sinks down (冘) is one that delays and loiters, hindering movement.
For タン, imagine a person delaying so long they get a tan: tan -> タン, and their body loiters in the sun.