脚 = ⺼ (ròu, flesh) + 却 (què) — a classic pictophonetic. The radical ⺼ is the "flesh form" of 肉 (ròu, meat), the same component that anchors the entire body-part vocabulary of Chinese: 脸 face, 腿 leg, 肚 belly, 肝 liver, 肺 lung, 胃 stomach, 腰 waist, 背 back. Whenever you see ⺼ on the left of a character, you are looking at a body part. The right-hand 却 supplies the sound (the modern reading què has drifted from the original jiǎo, but the phonetic relation is reconstructable in older Chinese).

Note the visual confusion: ⺼ (flesh) and 月 (moon, yuè) look identical in modern fonts. They are historically distinct components — the moon-radical sits in 明 míng "bright," 朝 cháo "dawn," 期 qī "period"; the flesh-radical sits in body parts. In handwriting some traditions distinguish them by the angle of the inner strokes; in print they have merged. Knowing the difference is one of the first quiet steps from beginner to intermediate Chinese: when you see ⺼ on the left of a character, default to body part.

The character carries two semantic lives that grew from the literal foot:

1. The body part. The everyday word for the foot of a human or animal — and, in much colloquial Chinese, the entire leg as well. (Standard medical Chinese keeps 脚 = foot and 腿 tuǐ = leg distinct, but spoken usage often blurs them.)
2. The base. Anything that has a "foot" in the metaphorical sense: 山脚 the foot of a mountain, 墙脚 the foot of a wall, 注脚 a footnote, 脚本 a script (the "foot-text," what the actor stands on).

The compound 足 zú is the literary and classical counterpart — and the actual Kangxi radical for foot characters (跑 to run, 跳 to jump, 跨 to stride). 脚 is the everyday colloquial word; 足 is the classical, formal, and radical-bearing word. Both mean "foot," but you walk on 脚 and you read about 足.

The metaphorical extension from foot of an animal to foot of any standing thing mirrors the English usage almost exactly — and is so natural across human languages that you can read most of these compounds the first time you meet them. Mountains have feet (山脚), walls have feet (墙脚), pages have footnotes (注脚), scripts have feet because they were originally read at the foot of the page (脚本). The character has quietly become Chinese's all-purpose word for the bottom on which something stands.

Picture 脚 as the flesh-foot: the ⺼ radical (a slab of meat, the body marker) on the left, the right-hand 却 carrying the sound. Then watch the metaphor walk outward: the foot of the body becomes the foot of a mountain (山脚), the foot of a wall (墙脚), the foot of a page (注脚), the foot of a script (脚本). Anything that stands in Chinese has a 脚 — the bottom on which it stands.

The pairing with literary 足 is worth holding: 脚 is what you walk on, 足 is what dictionaries are organised around. They mean the same thing; they belong to different registers. When in doubt in conversation, say 脚.