Stone-cold sober is a phrase that typically implies someone being completely sober from alcohol or drugs. However, there are various synonyms that can be used instead of "stone-cold sober". For example, "completely sober" or "100% sober" can be used to convey the same meaning. Another synonym could be "clear-headed" or "not under the influence". Additionally, "straight-edge" or "teetotaler" can be used to refer to someone who abstains from alcohol and drugs altogether. Ultimately, there are many different ways to express the idea of sobriety in language, and the choice of synonym will depend on the context and tone of the conversation.