The phrase "up for" is a colloquialism that means willing or ready to participate in something. Synonyms for "up for" include keen, eager, enthusiastic, interested, game, willing, prepared, and ready. These synonyms convey a positive attitude towards the activity at hand and suggest that the person is excited to participate. On the other hand, some synonyms for "up for" may include indifferent, reluctant, hesitant, apathetic, and uninterested. These synonyms convey a negative attitude towards the activity, suggesting that the person may not be fully invested or interested in participating. It is important to consider the context in which these synonyms are used to understand the speaker's attitude towards the activity.