Most sideways is not an adjective that has antonyms as it is a comparative phrase that indicates the degree of inclination or deviation from the normal. However, the word "sideways" can have antonyms such as upright, vertical, straight, perpendicular, or erect. These words refer to a position or direction that is opposite to sideways, which means a horizontally oriented object or person. Antonyms are words that have opposite meanings to a given word, and while most sideways may not have them, other words do, and it is important to understand their meanings to be able to use them correctly.