The phrase "sat one butt" typically refers to someone sitting down in a casual or lazy position. However, there are many alternative words and phrases that can be used to convey a similar meaning. For example, you could say that someone "plonked themselves down" or "sank into the chair." Alternately, you could describe someone as "lounging" or "lazing around." Other possibilities include "perched," "parked," "settled," or "nestled." Whatever the exact wording, these synonyms all suggest a relaxed, comfortable posture that implies a lack of hurry or urgency.