Fasten upon is a phrase that is commonly used to describe someone or something that has been fixated on. There are many synonyms to explore that offer a similar meaning. Words such as seize, latch onto, grab, attach, hold onto, grasp, hook onto, clamp, and affix all exhibit some degree of fixation. Each of these terms possesses their unique connotations and applications, depending on the context in which they are used. For example, while "seize" can convey a sense of urgency, "latch onto" suggests a more prolonged or persistent fixation. Ultimately, the choice of synonym will depend on the intended tone, subject, and context of the sentence.