The phrase "was on the lookout" implies a state of alertness or vigilance, actively seeking something or someone. Antonyms for this phrase would be words that suggest the opposite, such as carelessness, inattention, or indifference. Synonyms for these antonyms include disregard, neglect, and oversight. Other possible antonyms for "was on the lookout" include "was oblivious," "was unaware," "was not paying attention," "did not notice," and "was unobservant." Each of these phrases implies a lack of attention or focus, and can be used in contexts where the opposite of "being on the lookout" is desired or intended.