Although English '
kick' can sometimes serve as a noun and mostly as a verb, we know the noun '
kick' is derived from the verb '
kick' because etymologically speaking it is originally a verb, from Middle English
kyken 'to strike out with the foot', from Old Norse
kikna 'to sink at the knees' and
keikja 'to bend backwards'.
By the same token, although the character 子 can sometimes serve as a verb (e.g.,《史記》齊女子之) and mostly as a noun (e.g.,《荀子》聖王之子也), we know the verb 子 'to adopt as a child' is derived from the noun 子 'child' because it is almost always used as a noun in the oracle bone script.
there was no concept of parts of speech in ancient Chinese.
Old Chinese certainly has grammar or syntax (e.g., Subject-Verb-Object), from which we can determine the part of speech of a word in an utterance. That is to say, there actually are parts of speech in archaic Chinese. As a result, when I saw "there was no concept of parts of speech in ancient Chinese", I naturally interpreted it as "it was rather flexible concerning the part of speech of a word in ancient Chinese," like 子 and English 'kick' mentioned above. Despite the flexibility, we can still determine which one is a derivative and which one is the original, and "
it cannot be said that the parts-of-speech system in Old Chinese, specifically the period of the I century AD (Note: That's about the time when the pictogram 了 is first attested)
is absolutely flexible." "
The derivation of nouns from verbs is characteristic process of The Sino-Tibetan languages, whereas the reverse type is exceedingly rare." (Anastasia Durymanova,
Nouns and verbs’ syntactic shift: some evidences against Old Chinese parts-of-speech system’s flexibility). Since it is uncharacteristic, the derivation of verbs from nouns often involve tone change (i.e., 破讀, 字音詞化變音) in Middle Chinese or a hypothesized suffix -s in Old Chinese to remind listeners of a change in the part of speech, for example, original 雨 yu3 'rain' (noun) --> derivative 雨 yu4 'to rain' (verb); original 食 shi2 'food' (noun) --> derivative 食 si4 'to eat' (verb); original 蹄 ti2 'hoof' (noun) --> derivative 蹄 di4 'to kick in defence' (verb); original 衣 yi1 'clothes' (noun) ---> derivative 衣 yi4 'to wear' (verb); original 膏 gao1 'grease' (noun) --> derivative 膏 gao4 'to grease' (verb), to name just a few. There is no evidence from Middle Chinese that reflects a suffix -s for 了liao3 in Old Chinese.
Also, if "了" is originally coined as a noun, we would expect its early attestation to be mostly nouns. As it happens, it is rarely, if ever, used as a noun in its early attestation.
The dictionary says so because its synonym is also a verb. But how do we know this?
I didn't say "
the dictionary lists it as a verb because its synonym is a verb." I considered its dictionary entries and the part of speech of its synonym two independent justifications. It goes without saying that the assignment of part of speech by the dictionaries is based on attestation in actual texts (e.g.,《方言注》相了戾也).
How do we know its synonym 尥 is a verb? Because《方言》defines 尥 as "以足鉤之" and《說文解字》as "行脛相交".
Syntax: verb + pronoun 之
以足鉤(verb)之曰"尥"(verb)
Syntax: 相 (表示一方對另一方有所施爲) + verb
例如: 相送, 相信, 相煩, 相問.
《羣經音辨》足相躗(verb)曰"蹄"(verb)
行脛相交(verb)曰"尥"(verb)
《方言注》相了(verb)戾(verb)
