How to Sign "Why Not"
To sign "why not", begin with the sign for "WHY" by bringing your dominant hand near your forehead, palm facing inward, and pulling it away while forming a "Y" handshape. Then, smoothly drop your hand down and flip your palm to face upward in a casual shrugging motion. Your facial expression is key here—furrow your eyebrows if asking a genuine question, or raise your eyebrows slightly with a nod if you are agreeing to a suggestion.