The name of letter Ղ gave rise to the word ղատասություն ([ghatasutyun] – lallation),i.e. uttering of sound [gh] instead of [r]. Almost like the French pronouncetheir famous prance r.
In the present day Armenian language, letter Ղ stands for a consonant, intermediate between [g] and [kh]. But originally Ղ had utterly different phonetic meaning – it denoted a variety of sound [l]. This is evidenced by numerous borrowed words, in which Ղ took the place of [l]. Thus, the Greek word τάλαντον ([talanton] – talent) – became տաղանդ [taghand] in Armenian, and τιτλος ([titulus] – title) – was transformed into տիտղոս [titgos]. The same is true for words borrowed by other nations from the Armenian language. For instance, the Georgian word ([talavari] – pavilion, tent) – and the Armenian word տաղավար [taghavar].
When, how, and why did Ղ get to denote sound [gh]? It is believed to date back to IX century. According to linguist Hrachya Atcharyan, the alteration in the sound designated by this letter originated “in one of the gavars (regions), and then gradually spread throughout the country”. Having analyzed the words with Լ and Ղ, used by Armenianwriters of different centuries, the scholar concluded that in the beginning of IX century sound [gh] appeared in the north – it was used by the Armenians living on territories that bordered on the Caucasian Albania.
By the middle of the X century, it got down southward, and in the XI century it spread throughout the country. It is possible that the substitution occurred under the influence of languages of the neighboring nations. Especially since sound [gh] existed in all these languages, including Georgian, Albanian, the Byzantine version of the Greek, in Persian and Arabic. As to the shape of Ղ, taking into account the original phonetic meaning of the letter, its graphic form is believed to have originated in the Greek lambda (Λ) or the Armenian lyun (Լ).
See more at Aram Khachaturians "Armenian Alphabet. History and Symbolism" book