The Arm, palm down or cubit hieroglyph (Gardiner D42) has the phonetic value mḥ. A variant with the upper arm "slanted" is D41. It represents the Egyptian cubit (about 20 inches).
See also
Gardiner's Sign List#D. Parts of the Human Body List of Egyptian hieroglyphs Ancient Egyptian units of measurement#Length
References
Budge, (1920), 1978. An Egyptian Hieroglyphic Dictionary, p. 316a.