Legs that are a little bit more slim than usual (they can be super slim, but is not necessary as long they are slimmer than usual). The same way flat_chest and small_breasts is about breasts smaller than average, slim_legs describes the appearance and muscular mass of legs that are slimmer than average. As side effect, since anime/manga artists tend to draw legs with a bit more muscular mass than on real life (in order to make the characters look more sexy), slim legs can also being more realistic, just as side effect.

Examples:

This is an example of slim_legs:
Danbooru post #1392499

Same character, but this one is an example of what is NOT slim_legs:
Danbooru post #1384362

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