There are a number of antonyms for the word "outrivalling" that can be used to convey different meanings depending upon the context. Some possible antonyms include "falling short," "lagging behind," "trailing," and "losing ground." These words hint at a sense of lacking or inferiority, in contrast to the competitive and superior connotations of outrivalling. Other antonyms that might also be relevant include "collaborating," "cooperating," or "working together," which emphasize teamwork and joint efforts instead of individual competition. Whatever the context and desired tone, there are various antonyms for "outrivalling" that can be used to precisely convey the intended meaning.