1. To adopt a serious attitude or demeanor after having been joking or being silly. Often used as an imperative. Get serious for a minute, will you? This is not something to joke about.

2. To begin to seriously focus on something, especially after having been wasting time or procrastinating. Often used as an imperative. Your book report is due tomorrow—turn off the video games and get serious! I've been dabbling for a while, but I think it's time I got serious as a writer.

3. To realize or accept the truth; to be realistic or practical. Often used as an imperative. Oh, get serious—there's no way you got the job over the CEO's son, no matter how qualified you are.