Untackle, a relatively uncommon verb, means to not be able to tackle or overcome a problem or challenge. There are a few synonyms that can be used in place of untackle, depending on the context. Some possibilities include "unconquerable," "overwhelming," "insurmountable," and "impossible to overcome." Other options include "daunting," "intimidating," and "formidable." In some cases, the word "unmanageable" might also be used in place of untackle, especially if the issue is difficult to control or contain. Ultimately, the word chosen will depend on the specific situation and the desired tone or emphasis of the message being conveyed.