The antonyms for the word "more prankish," include serious, solemn, responsible, mature, and trustworthy. While "more prankish" implies mischievous behavior, the opposite end of the spectrum carries with it a sense of gravity and wisdom. Being serious, responsible, and trustworthy are traits that suggest a level of respect and reliability, which is the opposite of being prankish. As we grow older, we tend to lean towards the more mature end of the spectrum, becoming less interested in pranks and more concerned with the serious business of life. Thus, the antonyms for "more prankish" suggest a shift in personality towards maturity and responsibility.