了

According to the Shuowen Jiezi, a pictogram (象形) of a baby without arms. Compare with 子, where the arms are visible. The Shuowen defines 了 as "the crossing of lower legs".

In the Shuowen Jiezi Zhu, a commentary to the Shuowen Jiezi, it is said that the pictogram instead depicts entangled legs (了戾).

Its relation with the meaning "to finish" is likely via sound loan, but an argument can be made about "entangled legs" → "tie"→ "finish".

Unrelated to 亨.

Verb “to finish; to be completed” > perfective aspect particle (了₁, weakened form) > change-of-state modal particle (了₂). It eventually replaced classical 矣 (OC *ɢlɯʔ).

Two kinds of particle uses of 了 can be distinguished: the perfective aspect particle after verbs (conventionally written as 了₁) and the sentence-final modal particle (了₂). It is generally accepted (Wu, 1998) that these two uses of 了 are derived from the concrete verb “to finish”. The grammaticalisation of this verb had become common in the Tang Dynasty, initially in the form of ‹verb + (object) + perfective 了› to indicate the completion of an action.

The perfective particle subsequently underwent further grammaticalisation to become the sentence-final change-of-state modal particle; Liu (1985) has demonstrated that this last step may have involved the coalescence of sentence-final 了 with 也 in certain Mandarin dialects, as the pronunciations of 了₁ and 了₂ are distinct in these dialects, with 了₂ rhyming with 也.

Contrary to the suggestion in Schuessler (2007), this word is not related to Vietnamese rồi, due to the non-existent correspondence between any given phonemes. Thai แล้ว (lɛ́ɛo, “to be finished; already; then, afterwards”), Lao ແລ້ວ (lǣu, “to finish; to be completed; perfective particle”) are loans from Chinese.