A flexed arm that shows the biceps. It is often used in a direct meaning and an induced meaning. Direct meaning: everything related to strength, sports, fitness, etc. For example, if you are exercising at the gym, you can send this emoji to him. Derived meaning: about mental strength, determination, perseverance and ambition. It is also often used to express your support and encouragement to someone, like "I've got your back", "Go for it".
U+1F4AA
? Flexed biceps