The text says the meaning "sunglasses" is "entirely arbitrary", which is rather surprising in a book that is based on effective memory techniques.

Heisig's primitive naming system seems to have three main threads: a) based on the Japanese radical name; b) related to a previously-named primitive it resembles or is derived from; c) pictographic.

I think it's reasonable to rename 舛 (maiashi) because it doesn't look anything like dancing legs -- but only if you come up with something better.

I wonder if Heisig either forgot, or for some reason didn't want to explain his reasoning on this one.

Suggestions for memorizing "sunglasses":

1) (Pictographic): You can see a pair of sunglasses in there if you try, i.e. the two enclosed areas equate to the lenses. (Though perhaps a pair of sunglasses you accidently sat on might resemble the primitive more.)

2) (Imaginative): Imagine putting on a pair of sunglasses in the evening 夕, and then blindly bumping into the object on the right of the primitive.

The left radical is katakana TA "タ", while the weird radical on the right side is katakana WI "ヰ", so the primitive can be seen as TAWI タヰ. That in itself is curious, but pretty useless, unless you can think of a foreign word with those syllabes.

I think that Heisig's gave it the "sunglasses" nickname based on its form and not its utility for stories, as they look like a forced element. A helper is that medieval chinese judges blackened their glasses so that their eyes couldn't be "readed" during a trial, which makes it possible to assign to it a "judge/sheriff/law enforcer" meaning.

I personally use "thief mask" instead, it's funnier.