stumble, slip
躂 centers on losing one's footing: stumbling or slipping, often with a sudden loss of balance.
躂 combines 足 (foot) with 達, which likely contributes the sound タツ and the idea of movement, together suggesting stumbling or slipping with the foot.
The left side is the foot radical 足, and the right side is 達 (reach, arrive). Picture a foot trying to reach somewhere but stumbling and slipping instead.
For タツ, imagine a foot slipping on a tack: tack -> タツ, and the sudden slip makes you stumble.