measure, standard, criterion
矱 centers on a fixed reference for judgment: a measure, standard, or criterion against which things are evaluated.
矱 combines 矢 (arrow) with 蒦, which likely contributes the sound and relates to measuring or capturing. The exact historical development is uncertain.
An arrow 矢 beside a complex measuring frame 蒦: imagine using an arrow as a precise ruler to check a standard length.
For ワク, picture a wacky measuring contest where everyone uses arrows as rulers: wacky -> ワク, and the silliness helps you remember the standard.