Merriam-Webster's Collegiate® Dictionary, 11th Edition
FLEX
I. verb
Etymology: Latin flexus, past participle of flectere to bend
Date: circa 1521
transitive verb to bend especially repeatedly,
2. to move muscles so as to cause flexion of (a joint), to move or tense (a muscle) by contraction, use , demonstrate , intransitive verb bend ,
II. noun
Etymology: short for flexible cord
Date: 1905
an electric cord,
III. noun
Date: circa 1934
an act or instance of flexing