挑 challenge

If you consider an omen to be a kind of sign, then you can say:

Gangsters use hand-signs to challenge other. This is known as “hitting up" or “waving hi."

challenge ★★★☆☆
挑 (challenge) + 戦 (fight) = 挑戦 (challenge)

challenge (challenge yourself to do better, test your limits / challenge the system) be defiant or provocative. take the pepsi challenge!

a contender, challenger ☆☆☆☆☆
挑戦 (challenge) + 者 (professional) = 挑戦者 (a contender, challenger)