Short Answer
The shapes「夂」and「夊」are writing variants. As a component of a character, the majority of the time it is just a「止」(picture of a foot) pointing in the opposite direction to「止」.
The dictionary definitions are tangential to the component meaning.
- 「夊」is defined as walking slowly, an extended meaning from「止」, which represents motion in general.
- 「夂」is defined as arriving from behind, but this was probably made up. Apart from the single Shuowen definition, which all other dictionary definitions seem to stem from, there are no recorded words/expressions/phrases which contain「夂」meaning arriving from behind.
Long Answer
Is there a difference between 夂 and 夊?
The question is complicated through differing ways of writing these components throughout history, to this very day, often due to confusion as to what「夂」and「夊」actually are. In any given character, different fonts or language standards may use one or the other, or even use the unrelated「攵」.
「攵」is completely unrelated to「夂」and「夊」. For reference,「攵」originally depicted a hand「又」holding a beating implement (stick, whip, staff).「攵」, or its variant「攴」, is heavily used as a semantic component to do with disciplining, attacking, or more generally motion.
Shape evolution of the「攵」component, as shown in later forms in the character「教」. The character「教」depicts a child「子」learning arithmetic (represented by two「㐅」, original character of「五」) under threat of being beaten by「攵」. The two「㐅」and「子」later fused into「孝」; the original meaning is better preserved in the Kangxi variant「敎」. The second form above shows the shape similarity to「攴」; here, the beating implement changed slightly to the shape of「卜」(Zhengzhang OC: /*poːɡ/) to provide a phonetic hint for「攵」(Zhengzhang OC: /*pʰoːɡ/, /*pʰroːɡ/).
Other examples of the use of「攵」include:
- 「牧」, which depicts a cow「牛」being herded by a hand with a whip「攵」;
- 「變」(change, Zhengzhang OC: /*prons/), phonetic component「䜌」(Zhengzhang OC: /*b·roːn/, /*b·ron/, /*b·rons/);
- 「做」(to make or do), both semantic and phonetic component「作」corrupted into「估」;
- 「改」(change, Zhengzhang OC: /*klɯːʔ/), phonetic component「己」(Zhengzhang OC: /*kɯʔ/);
- 「攻」(attack), phonetic component「工」.
「夊」was originally「止」(a foot), but pointing in the other direction:
Compare「止」:
Here's where things get tricky.
「夊」did not originally represent a word; it was not an individual character (being found exclusively as part of other characters), nor appear to have any pronunciation, until the Qin-Han era. The pronunciation recorded in dictionaries, 楚危切 (Mandarin suī), was a word meaning something like slow strides, variously represented as「綏」or「奞」. For all intents and purposes, as part of other characters, it is just a variant form of「止」.
The shape of (not the word represented by)「夂」is just a variant way of writing「夊」. Note that the stroke「㇏」is just for drawing the big toe and the outline of one side of the foot without the other toes.
The difference in the shapes is trivially just whether the stroke「㇏」protrudes or not, and both shapes are seen, for example in「各」:
This indicates that「夂」and「夊」are just variant ways of writing upside-down「止」. There was no such rule saying that the stroke must protrude when the component is on the bottom of the character and must not protrude when it's on the top; this is a convention made up later, possibly out of confusion about what「夂」and「夊」actually are, or possibly for aesthetics.
Despite their modern appearances, characters which contain「夊」on the bottom, such as「後」, have no requirement that the stroke must protrude.
Shuowen confusingly lists a separate character with an identical shape to「夂」, and gave it a definition 从後至也 (approaching from behind) and pronunciation 陟侈切 (zhǐ). This definition was probably made-up, and is suggested to be a Simplified form of「致」(from where its definition and pronunciation was derived). For reference,「致」was originally comprised of「至」(both a semantic and phonetic component) and「人」(a semantic component); later on,「夊」was added and「人」was removed.
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集成2838秦
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睡ㆍ雜35
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It is surmised that scholars didn't know what「夂/夊」was doing in this character, having not known that「人」was omitted previously, so forced a Shuowen definition into existence in an attempt to explain its presence in「致」.
Shuowen also mentions「夂」in its entry for「冬」:
四時盡也。从仌从夂。夂，古文終字。
This corresponds to some dictionaries listing the pronunciation zhōng. This component has an entirely different history, originally depicting objects hanging off the end of a connector, indicating the meaning end.
A line was later drawn across the character, bridging the two branches. To emphasise the meaning winter, sometimes「日」was added to emphasise time > seasons, and other times「仌/冫」was added to emphasise cold or ice:
However, the crossing line made the shape prone to corruption into「夂/夊」, which leads on to the modern form.
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References:
