Forthwith is an adverb that means immediately or without delay. Synonyms for forthwith include promptly, directly, straightway, instantly, and immediately. Other synonyms that convey the same sense of urgency include pronto, right away, posthaste, right now, on the spot, and at once. These words are typically used when someone needs to act quickly or when an urgent situation arises. In literature, the use of specific words like forthwith and its synonyms can add tension or urgency to a story. By using synonyms interchangeably with forthwith, a writer can create variation and maintain reader interest.