KanjiJLPT N4
Leg

leg, foot, be sufficient, counter for pairs of footwear

Mnemonic

Your leg has a mouth on top and a shoe on the bottom. The mouth constantly nags the shoe: "Have we walked enough yet?" The leg won't be sufficient until mouth decides it's satisfied—only then does the shoe finally rest.
Additional thoughts
Picture 足 as a mouth (口) stacked over a shoe-shaped base. The mouth is the judge of sufficiency, deciding when enough steps have been taken. Leg = mouth + shoe, and 'enough' is baked right into the character.
Quick recall
Your leg is a mouth atop a shoe, nagging until it's walked enough.

Details

The keyword for 足 is leg. This kanji refers to the leg or foot, encompassing the entire lower limb used for standing and walking. Beyond its anatomical meaning, it also carries the extended sense of being sufficient or enough, as in having one's needs fully met. These two meanings—the physical limb and the concept of sufficiency—are both central to this character's usage.
On'yomi
そく
Kun'yomi
あし、た.りる、た.る、た.す