The Hunmin Jeongeum Haerye, the treatise introducing the principles behind the Korean alphabet written by its inventor King Sejong in 1446, explains that this glyph was derived from the "outline of the tongue" when pronouncing this "semi-lingual sound" (半舌音), but does not elaborate.
Gari Ledyard suggests that Sejong may have had his inspiration for ㄹ from the 'Phags-pa letterꡙ(l), although he is more tentative here than with the Hangul letters for the plosives. Ledyard gives evidence that Sejong was inspired by 'Phags-pa for the basic glyph forms, although he changed the shapes of the letters drastically in order to enhance the simplicity and rationality of his script, and the ultimate shape of the letters may indeed have been influenced by that of the speech organs (Ledyard 1997).
From Middle Koreanㅭ(-lq), from Old Korean尸(*-l). The Old Korean form (up to the thirteenth century) was primarily an irrealis verbal nominalizer, rather than an adnominal suffix as it is now. In fifteenth-century Middle Korean both the nominalizing and adnominal functions were in use, but the nominalizing function was already quite archaic. Since the sixteenth century, the suffix has only had an adnominal meaning.
After consonants, the allomorph -을 (eul, “-eul”) is used.
-ㄹ (-l) is appended to the sequential form. Similar to a future participle, the resulting determiner indicates that the referent of the following substantive will perform the action described by the verb to which -ㄹ (-l) is attached:
가다 (gada, “to go”): 가니 (gani, “ga-”) + ㄹ (l): 갈 (gal, “who will go; that sb will go to/at/by...”)
묻다 (mutda, “to inquire”): 물으니 (mureuni, “mureu-”) + ㄹ (l): 물을 (mureul, “who will inquire; that sb will inquire”)
날다 (nalda, “to fly”): 나니/날면 (nani/nalmyeon, “na-/nal-”) + ㄹ (l): 날 (nal, “that will fly”)
The suffix -ㄹ (-l) is frequently used along with several dependent nouns, such as 것 (geot, “thing”) and 때 (ttae, “time”), for grammatical purposes. As an irrealis mood marker rather than a tense marker, it can either denote the future tense, or nothing:
(future, intention) 그 친구는 이번기회에 고향집을 찾을것이라고 했다. (Geu chin-guneun ibeon gihoee gohyangjibeul chajeul geosirago haetda., “He said he will visit his hometown by this chance.”)
(future, guess) 물이 차가울것같다. (Muri chagaul geot gatda., “I guess the water is cold.”)
cf. 물이 차가운 것 같다. (Muri chagaun geot gatda., “The water seems cold.”) / 물이 차가웠을것같다. (Muri chagawosseul geot gatda., “I guess the water was cold.”)
물이 차가울 것 같았다. (Muri chagaul geot gatatda., “I guessed the water was cold.”) / 물이 차가웠을 것 같았다. (Muri chagawosseul geot gatatda., “I guessed the water had been cold.”)
(no tense) 그가 하는말중에 믿을만한 것이 있을리가 없다. (Geuga haneun mal jung-e mideul manhan geosi isseul riga eopda., “Among the words he says, what is worth believing cannot ever exist.”)
(no tense) 목발은 발이나 다리를 다쳐잘걸을수 없을때사용한다. (Mokbareun barina darireul dachyeo jal georeul su eopseul ttae sayonghanda., “We use crutches when we don't walk well due to foot or leg injuries.”)