The Hanuman Chalisa was composed by Goswami Tulsidas in Awadhi and takes its name from its forty chaupais, which are framed by two dohas at the opening and one at the close — forty-three verses in all. It recounts Hanuman's strength, his learning, and his devotion to Rama. It is recited widely from memory, on Tuesdays and Saturdays in particular.
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The Hanuman Chalisa is named by its length. Chalis means forty, and the text runs to forty verses between its opening and closing couplets.
Source: Hanuman Chalisa, Tulsidas (Awadhi)