/ ˌfɔːθˈwɪθ; -ˈwɪð; NAmE ˌfɔːrθ-/ adverb
( formal ) immediately; at once :
The agreement between us is terminated forthwith.
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WORD ORIGIN
Middle English (in the sense along with, at the same time ): partly from earlier forthwithal , partly representing forth with used alone without a following noun.