"Tekeli-li" comes originally from
Edgar Allan Poe's "
The Narrative of A. Gordon Pym", where it is the noise a bird in the polar land of the Tsalal makes.
Obviously, H. P. Lovecraft used it as inspiration for his first and only true appearance of the Shoggoths, in his story At the Mountains of Madness. It is thought that the Shoggoths cried "tekeli-li" to mimick the sounds that the Elder Things made.