The word "out-side" refers to the exterior or external part of something, and as such, its antonyms would be related to the opposite meaning. For instance, inside, interior, within, and internal could all be appropriate antonyms for "out-side." Moreover, other related antonyms could include the terms inner, core, center, contrary, opposite, or contradictory. Depending on the context, other antonyms that may apply might include intrinsic, innate, or inherent, which would refer to qualities or characteristics that are part of something's nature, rather than its external appearance or location. In summary, the antonyms for "out-side" are those terms that convey a sense of interiority or opposition to what is external or superficial.