Middle Primitive Elvish
taikā
adjective. steep, tall, deep
stal
root. steep
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A root in The Etymologies of the 1930s glossed “steep” with derivatives like Ilk. thall “steep, falling steeply (of river)” and Ilk. thalos “torrent”, the latter used for the river name Ilk. Thalos (Ety/STAL). Tolkien continued to use the name S. Thalos in later versions of The Silmarillion, but the name was translated nowhere else, making its continued connection to the 1930s root uncertain.