The phrase "waiting concealed" suggests someone or something is hidden or lying in wait. Synonyms for this phrase could include words such as lurking, hiding, ambushing, or crouching. The context of the phrase would determine which synonym would be most appropriate. For example, if the phrase were used in a hunting context, "lying in wait" or "ambushing" might be suitable synonyms. However, if the phrase referred to a person who was hiding, "lurking" or "concealing" might be more appropriate. Overall, there are many synonyms that could be used for "waiting concealed," depending on the situation and context in which the phrase is used.